So far as I can tell, to summarize, the house rules change two things about item creation:
- An item creation feat's prerequisite is not caster level but, instead, effective character level equal to the feat's caster level. For instance, the feat Brew Potion has a prerequisite not of caster level 3 but of ECL 3.
- Spells that are prerequisites for creating a magic item need not be supplied every day of a magic item's creation, but, instead, only once and at the end of the magic item's creation. For example, a creature that wants to brew a potion of haste [trans] (PH 239) (3rd-level spell at caster level 20) (3,000 gp; 0.1 lbs.) need not supply the haste spell three times (once per day for each day necessary to create the potion) but, instead, only once.
I don't think these house rules will have much impact on the PCs. That's because my experience has been that players find the idea of magic item creation exciting but, in actual play, find the process uninteresting and inconvenient. It's interesting to quest for unusual raw materials or to secure obscure spells that will be used in item creation—those're adventures!—, but sitting around waiting for the metaphorical pot to boil is boring. Likewise, it's more convenient to buy magic items—price be damned!—because creating the magic item from scratch expends more of the PCs' resources than just buying the magic item outright (e.g. money versus feats, raw materials (i.e. "Still half the money!"), time, and spells—if that last is even a significant concern; see below).
Consider the dedicated wright
The only workaround I've seen PCs employ that encouraged item creation is a PC using a dedicated wright (Eberron Campaign Setting 285) because then item creation didn't slow down the PCs! And consensus seems to be that when a magic item is created by a dedicated wright, the second bulleted house rule already applies—except that the prerequisite spells are cast at the process's start rather than at the process's completion. (See here.)
War Story
In one campaign a PC had many item creation feats, but the PCs consistently put themselves on too tight of a clock to ruthlessly exploit the item creator's item creation abilities. It wasn't until the PCs were levels 19 and 20—the campaign nearing its end—that the item creator had time enough to do awesome stuff. Even then, though, it was easier to purchase, for example, several attuned gems of wish [univ] (PH 302-3) (9th-level spell at caster level 17) (33,100 gp; 0 lbs.) from the City of Brass than it was to wait over a month for the craftsman—or his wright—to make one.
So for the 2,100 gp price of a dedicated wright—assuming the DM allows a creature that participates in its creation to be considered the dedicated wright's owner (not a big assumption; Pathfinder, for instance, describes how noncasters get homunculi)—, a creature can essentially take itself off the item creation clock and put his servant on it instead so that item creation occurs, essentially, in the background, off-screen not on-screen. Further, the item's real creator need only cast the magic item's prerequisite spells once. (In the War Story campaign, the PCs outfitted an enveloping pit (Magic Item Compendium 159) (3,600 gp; 0 lbs.)—one of the game's best deals—with a laboratory suitable for item creation and carried the pit with them, dedicated wright slaving away within, creating magic items whilst they adventured.)
That 2,100 gp is a sum small enough that I could imagine just awarding the effects of a dedicated wright to all PCs at levels 7 or 8 even in an otherwise normal campaign—like a house rule saying that a caster that possesses the appropriate item creation feat and that casts each prerequisite spell once on a pile of appropriate raw materials eventually (i.e. when the appropriate normal amount of time for item creation has passed) causes the raw materials simply to transform into the desired magic item, but a caster can only have one magic item in progress at a time. I don't foresee this Easy-Bake magic item process significantly harming the campaign: the pieces are already there. Making this house rule available to everyone at level 1 is more significant, but wizards already win D&D, so it's not a big deal if wizards win a little harder while everybody else gets a leg up, too.
"Item creation feats for some, miniature American flags for others!"
I'm not sure opening up the item creation feats to any creature is the best choice. It's been my experience that those creatures that don't have access to item creation feats are already the most feat-starved. While a rogue would love to, for example, craft his own wondrous items, he's barely doing traditional rogue stuff with his tiny number of precious feats! I suspect you'll find opening up item creation feats to noncasters means there may be one extra item creation feat in a traditional party of four, and I suspect the item creation feat may be one you didn't expect, like a barbarian taking Tattoo Magic (Races of Faerûn 170) so that he can make inexpensive magic items he can activate while he's raging, or a rogue taking Craft Drilbu (The Mind's Eye Web column "The Crystalsinger (Prestiege [sic]
Class)") so as to push the limits of magic–psionic transparency. In short, this, too, is not a big deal… on an individual scale.
On the campaign scale, however… wow. Remember: commoners get feats. A level 1 human commoner can now take the feat Scribe Scroll (PH 99-100)—the universe yawns—, but then that same level 1 human commoner can take any one of the feats Exceptional, Extraordinary, or Legendary Artisan (ECS 52, 53, and 56, respectively) to significantly reduce the time, price, or XP cost of magic items he helps to create! Suddenly, those level 1 human commoners are extremely popular with all the folks who can actually make magic items! Such a campaign should see magic item prices in general drop by 25% and, I suppose, an overall rise in the standard of living for pretty much everyone. And that's just to start. Like I said, this isn't particularly important to the PCs, but to maintain the setting's verisimilitude the setting must accommodate this increased access to item creation feats… especially those that don't, per se, actually create items!
With all this in mind, I'm not sure your house rules accomplish your goals. I've already mentioned how this change likely should make magic items in general less expensive. Further, the game already allows the DM to change or nix—out of hand, for whatever reason—magic items, spells, or anything else the DM doesn't like, but the house rules loosen the DM's otherwise iron grip. That is, while it normally takes a number of scrolls equal to each 1,000 gp of the item's base price to make a spell completion or spell trigger magic item, instead that one scroll can make of that spell a wand, scepter (Lost Empires of Faerûn 8), staff, or even eternal wand (Magic Item Compendium 159-60) or minor schema (Magic of Eberron 47), turning that one scroll into fifty or nigh-infinite copies of that spell! That spell the DM only put one scroll of in the campaign? Once it's found, it'll be out in the wild in short order, its DRM totally hacked. A campaign like this should have tens of thousands of crazy, niche spells because spell knowledge can spread so fast. That sounds fun and fascinating to me, but I'm not sure that's what you're looking for. (But for comparison also see the obscure 6th-level Sor/Wiz spell the Hamagess' staffsprout [trans] (Mintiper’s Chapbook Web column "Part 10: Chronicler’s Compedium").)
Finally, if a goal is to make crafting magical items accessible to any class (though not necessarily any build), that's goal's achieved, but making something accessible doesn't necessarily make something desirable—a fighter will still likely find his feats better spent on fighting and a rogue's on… doing rogue stuff. They need those feats doing what those feat already do to do what those characters need to do! To really test the long-term impact of this—which I haven't done—, a house rule would need to be made that says something like for every three, five, seven, ten or whatever levels a creature takes in classes that don't cast spells—maybe even including the first—the creature gains a bonus item creation feat. Then see what folks pick and how that changes the campaign.
An aside: nuking item creation feats altogether
When I first read the question, I thought the house rules also eliminated item creation feats—a fairly radical change but one not without precedent. That is, the Forgotten Realms supplement Silver Marches includes a magic college, the Conclave of Silverymoon (62-3), where a creature that attends can supply the cash, spells, and XP but a hapless undergrad the item creation feat. The cost for this service? Ten percent of the magic item's price. (I should note that to receive this benefit—and several others—a wizard pays per year 500 gp plus 500 gp per wizard level above 1 up to a maximum of 4,000 gp for a creature with 7 levels of wizard—a pricing scheme complicated enough to rival actual college. While this may sound like a lot of cash, amortizing this over a year to 77 gp a week should make it affordable in all but the most parsimonious of adventurers.) In other words, completely eliminating item creation feats and increasing magic item prices by 10% (plus, perhaps, a consummate increase in the cost of living?) is another possibility to consider, although such a change—unsurprisingly since it comes out of the Realms—hurts noncasters much more than it helps casters.
Best Answer
Note: I'm not experienced at balancing homebrew content. I am simply comparing these items to official (DMG) material assuming it is balanced.
Item 1 is overpowered
Anti-magic field is an 8th level spell. an 8th level one-time use scroll according to DMG page 200 is a very rare item. So one that refreshes every day should be at least legendary, but even that might be too powerful.
Item 2 is inconclusive
there are several wands, such as wand of fireball that have up to 7 charges of 3rd level spells and recharge 1d7 of them at dawn, so that would suggest that 3rd level counterspell that has only 4 charges and recharges 1d4 of them would be underpowered. But that does not take into account action economy - to use wand of fireball you have to use an action, but counterspell can use reaction. Given the fact that most of the time most characters don't get an option to use reaction that makes this item much more powerful. I haven't found a good analogue for this in official materials, so the value of 3rd level spell charges vs 'free' reactions is debatable.
Item 3 is at least rare
The closest analog to this is Mantle of spell resistance which is rare and states:
First we have to evaluate the difference in benefit on all spell saving throws vs Dexterity saving throws. If we assume that spells are equally likely to use Dexterity, Wisdom, Constitution or one of the other stat saving throws that means this artifact is about 1/4rd as good as Mantle of spell resistance.
Second we have to evaluate the relative power of the effects. If the saving throw is likely to succeed 50% of the time, this item will reduce damage by average of 75% (50% on fail and 100% on success) while the Mantle of spell protection will only reduce damage by average of 12.5% (25% of time(if first roll fails but second succeeds) by 50%). That makes the effect of this item 6 times stronger than Mantle of spell protection.
As a result this seems to be about 50% stronger, but less versatile item than Mantle of spell resistance, so it should be at least as rare if not more so.
Item 4 likely underpowered
Currently it does not specify DC of the saving throw, that makes it impossible to tell how often this ability will come into effect, but generally if you are fighting in a party or with minions and the caster has more than one enemy in range, the usefulness of this ability would be quite limited, because choosing the second best target for a spell is usually almost as good as choosing the best target. There might be some enemies that have only limited ranged spells, but even then the fact that the effect and on any aggressive action mean that the item is useful in only very niche circumstances.
Item 5 is probably ok
As you said there are no equivalents of this item anywhere in the official rules as there are no items that guarantee breaking concentration, especially not for a group of creatures. But there is a spell Magic Missiles which makes the target roll 3 concentration checks without any chance to avoid them, and you can get it from uncommon item Wand of Magic Missiles with 7 charges 1d7 of them recharging each day. Changing 3 concentration rolls to guaranteed failure, removing damage, but giving area of effect (which might not be that useful unless casters are plentiful) while increasing rarity seems quite fair.
Brooch of shielding is balanced
My method of evaluation is comparing items to official content. As Brooch of shielding is official all I can say is that it is balanced
Item 7 gets better with tougher foes
Evaluating this item is tough because while it seems very similar to armor of resistance and magic weapons seem to be about as frequent as some of the damage types the problem is that magic weapons are mostly possessed by higher level opponents unlike damage types which are distributed more randomly or based on theme. That means that this kind of armor would be next to useless for one facing low level mundane enemies such as brigands and orcs, while being extremely useful in battles against fiends and high level adventurers (unless they have backup non-magical weapons).
Item 8 seems close to balanced
Looking at Legendary items that provide protection against spells there are Ring of spell turning, Robe of Archmagi and Scarab of protection - all of which give advantage to saving throws, but also have additional effects. While resistance is clearly superior when targeted by spells that are used primarily for dealing damage, most of them can also be partially mitigated by succeeding on saving throw while resistance does nothing against crippling debuff spells. In the end resistance vs advantage on saving throws seems quite close, so this item seems quite balanced.