It's Caster Level 8
Your second interpretation is correct. Here's the rule itself:
Creating a magic weapon has a special prerequisite: The creator’s
caster level must be at least three times the enhancement bonus of the
weapon. If an item has both an enhancement bonus and a special ability
the higher of the two caster level requirements must be met.
Then from the Magic Weapons page:
In addition to an enhancement bonus, weapons may have special
abilities. Special abilities count as additional bonuses for
determining the market value of the item, but do not modify attack or
damage bonuses (except where specifically noted). A single weapon
cannot have a modified bonus (enhancement bonus plus special ability
bonus equivalents) higher than +10. A weapon with a special ability
must have at least a +1 enhancement bonus.
From here, it's pretty clear that enhancement bonus and special abilities are distinct things. You combine them for the weapon cost (as the rule stats), but not for the crafting caster level requirement as each type has its own requirement. You just have to be able to meet all the requirements.
Thus, you need CL 6 (3x2) to make a +2 enhancement bonus, and CL 8 (and have access to the spells Chill Metal or Ice Storm) for Frost.
This also means all the weapons on the chart can be created without being epic level, as if you needed to be CL 9 to make your +2 Frost Weapon, it'd be impossible to do any combination that is >= +7 without epic levels. (You can't make a +7 sword without epic levels, but you can make a +5 Keen Frost one, which has the cost of a +7 equivalent weapon.)
It Could Be Caster Level 6
Depending on how you want to interpret the rules around prerequisites, you could create the weapon at CL 6 as a Druid (who get access to Chill Metal at level 3). The reason for that is these rule passages. From the SRD:
While item creation costs are handled in detail below, note that
normally the two primary factors are the caster level of the creator
and the level of the spell or spells put into the item. A creator can
create an item at a lower caster level than her own, but never lower
than the minimum level needed to cast the needed spell.
And from DMG p. 215:
For other items [besides potions, scrolls, and wands] the caster level
is determined by the creator. The minimum caster level is that which
is needed to meet the prerequisites given.
Depending on how you read this, the prerequisite could only be access to the spell itself, rather than the CL listed. In that case a level 6 Druid could create the item (as they can do both +2 and have access to a required spell). A Sorceror would need to be level 8, as that's when they get access to Ice Storm.
I'm not 100% convinced this is how they intended it to work, but it makes an item entry like Universal Solvent make a lot more sense. It has a listed CL of 20, which for such a common an inexpensive item just seems really wonky if only the greatest spellcasters on the planet are able to create it (particularly since the required spell is available to Wizards at level 9).
If you use this interpretation, the CL listed on the item entries is a guideline, or what you'd expect a normal item of it's type to be if you find one in the world.
1) Yes, you can.
The only exception are:
- Metamagic feats, you need the proper feat to craft each type of item.
- Spell Completion items, you need to know the spell to write a scroll.
- Spell Trigger, you need to know the spell contained in the wand or staff.
Everything else you can simply increase the DC by 5 points per requirement missing.
Using your example, the Ring of Gate has a normal DC of 27 (10 + item's caster level, 17), if you know the gate spell. If you don't know the spell, or if you cant pay for someone to cast it every day spent crafting it, then you must increase the DC to 32.
2) You don't have to provide all requirements by yourself, you can get help from others. Example: a cleric could craft a Wings of Flying with the help of a wizard. But everyone helping you must be present every day spent crafting the item, otherwise the process must be started again.
That said, yes you can make any item, even if you don't meet the requirements, the increased DC means you have to take different approaches, research a different ingredient to replace the missing spell and stuff like that.
3) No, the caster level is the default DC for every item, it is also used to roll saving throws for unattended magic items, and to define the spellcraft DC to identify them (15+CL).
To craft a Ring of Gate, all you need is enough spell ranks to beat the DC 32 check and have the Craft Wondrous Item feat.
Best Answer
Yes, increasing your caster level does allow you to meet specific prerequisites earlier.
However, these increases would need to be increases to your caster level for that class as a whole (or increased for crafting), not increases in caster level for a specific spell. Here are a couple of examples:
This would not apply because the caster level increase would not be applied to crafting.
This would apply, since the caster level increase is applied at all times (and thus applies when crafting).
This would not apply as it doesn't last the 8 hours of crafting, and only increases your caster level for casting spells and not crafting.
However, typically you don't need to meet the caster level prerequisite of an item in order to craft it.
From the Magic Item Creation Rules:
Therefore, if an item has a caster level as a prerequisite for crafting it, you can ignore it for a +5 to the DC of crafting that item.
Note that while the base DC to craft a magic item is 5 + that items caster level, there is not usually a caster level prerequisite for a magic item (and even if there is one as a prerequisite, it may not match the item's caster level).
This means that for a level 5 wizard with Craft Wondrous Item and Haste, despite Boots of Speed having a caster level of 10, the DC for crafting them is only 15 (base 5 + caster level of 10). It is not increased to 20 because there is not caster level prerequisite for crafting it.
The exception to this is constructs, which you use as an example.
From the Construct Creation Rules:
Therefore, in order to craft a construct you need to meet any caster level prerequisites it lists in it's crafting requirements.
Here are a couple items that can increase you caster level for crafting a construct.
Golem Manual: Golem manuals are one time use items crafted for a specific golem and increase your caster level golem crafting that golem. Note that unlike constructs whose caster level requirement you can't bypass, you can bypass the caster level requirement of a Golem Manual as it's a wondrous item.
Automaton Core: You can get an automaton core by taking Improved Familiar, selecting an Automaton Familiar, and murdering it, thus obtaining it's core.† A spellcaster holding an automaton core treats her caster level as 4 higher for all purposes. This allows you to use it for crafting as well as casting.
†: I am not responsible for any consequences you GM may give you for doing this.