As the RAW you quoted yourself indicates, the gauntlet should be considered exactly as an unarmed attack except that it deals lethal damage. The 1d3 damage is just the normal unarmed attack damage, and it is provided as a base value, thus Superior Unarmed Damage would apply so long as you meet the prerequisites. About the +5 gauntlet, I wouldn't bother... it's much easier and better to get a +5 Necklace of Natural Weapons.
You cannot wear armor that is not sized for you
Most items are allowed to work when missized, and weapons work with a penalty, but no such rule exists for armor, and armor is referred to as having a particular, fixed size.
Armor spikes don’t necessarily have to be the same size as the armor they’re on
There simply isn’t a rule anywhere that says so.
At first glance, this seems absurd and an oversight, but I think a case can be made for “extra large” spikes. This is most notable in the case of armor made for half-giants and goliaths, as their Powerful Build means they are somewhere halfway between Medium and Large, and can use Large weapons without penalty though they wear Medium armor. For a goliath, it makes sense that they would wear Medium armor with Large armor spikes.
The handedness of armor spikes is undefined
Armor spikes are supposed to be Light weapons, do not get used in any hands, and at no point do the rules address over-large spikes directly. In theory, Large armor spikes on a Medium creature would now be one-handed weapons and thus eligible for wielding “in two hands,” but there is no way to actually use two hands while wielding them. Thus, I do not think it is possible to get the 1½ Str to damage or the 2:1 Power Attack ratio with this method. They do get the Str to damage and 1:1 ratio of a one-handed weapon, because that is what they are. The rules do not actually say that one-handed weapons get those while “wielded in one hand,” unlike the two-handed versions.
But you could take things a step further: Huge armor spikes on a Medium creature are two-handed weapons. As with one-handed weapons in one hand, two-handed weapons don’t actually specify that you only get the improved damage when wielding them in two hands, because that’s assumed. The text is “two-handed weapons and one-handed weapons wielded in two hands.” So in this case you would get improved Str to damage and Power Attack. And a −8 penalty to attack for wielding a ludicrously oversized weapon.
The armor still does not take up any hands, in any of these cases.
This really does not seem like the right way to go
You have a monster that’s given a higher CR than it probably deserves, against PCs nearly twice that CR in the first place. The sensible thing to do here would be to houserule them into something better, or at the least give them class levels (I recommend Fighter 6 with the Dungeoncrasher ACF from Dungeonscape, or else Crusader or Warblade). This particular collision of the rules is wonky, and still will not provide particularly effective combatants.
Best Answer
No, the gauntlets are listed as weapons and no indication is ever given that they interact with the Hands slot.
I would, in most cases, treat them similarly to shields, however: single physical item, separate mechanical stats for use as a weapon and use as non-weapon equipment. This is just the same as shields, which point out that their Enhancement bonus to AC does not apply to shield bash attacks, but shields can be enhanced separately as magical weapons in their own right, in addition to their armoring properties. The rules do not specify this, however, and by RAW the two are completely independent such that you can equip both at once.
Gauntlets are explicitly listed as “Unarmed” weapons, which implies that if you drop your weapon (a Free action) and are now unarmed, but are wearing gauntlets, you should use gauntlet damage. Absolutely no need to draw anything.
Weirdly, spiked gauntlets are listed as “Light Melee” weapons, which implies that they work more like regular weapons, including the need to draw them and the inability to use another weapon while you have a spiked gauntlet equipped and “drawn,” whatever that means. Spiked gauntlets also use a separate entry that does not reference the gauntlet in any way, so we cannot equivocate the two that way. All that said, this is ridiculous; I would just treat them just as gauntlets that do piercing damage and have slightly higher base damage. After all, that’s all they actually are.
Yes and yes. You can also use the Two-Weapon Fighting combat maneuver with them.
Gauntlets, yes, spiked gauntlets per the above, not-by-RAW-but-that’s-ridiculous.
I could see houseruling to prevent things like eager and warning though. Something like “any magical weapon properties of your (spiked) gauntlet are suppressed while you are using that hand to hold another magical weapon.” It’s not in the rules though.
Very ambiguous. The text of the gauntlet is as follows:
The question is exactly what “otherwise” refers to here. If it is just the lethal aspect of the damage, which I personally find the most grammatically sensible and the most balanced at the table, then yes, the gauntlet is literally in all ways identical to an unarmed strike, except that it deals lethal damage.
But even that leaves some concerns. For example, you cannot magically enhance your unarmed strike: if a gauntlet is in all ways identical to an unarmed strike except that it does lethal damage, does that mean you also cannot magically enhance gauntlets? Are unarmed strikes and gauntlets considered the same weapon for, e.g., Weapon Focus? Do gauntlet attacks provoke Attacks of Opportunity if you lack Improved Unarmed Strike? The term “otherwise” here is dangerously broad in scope, after all.
Another reading is that the “otherwise” refers to the entirety of the gauntlet’s statblock: the name (i.e. it is a separate weapon), the damage, and so on. This would mean that no, bonuses to/improvements of unarmed strike damage do not apply to gauntlets, Weapon Focus (unarmed strike) and Weapon Focus (gauntlet) are quite separate, and so on. But then the question becomes, “in what ways is the gauntlet like an unarmed strike? What’s left?” I can’t really think of anything.
The way I rule it is this:
I houserule it so that there is a way to magically enhance unarmed strikes, either by having the warrior be “worked on” by the crafter or by them equipping some kind of item that does it (“hand wrappings,” “gloves,” whatever).
The gauntlet and spiked gauntlet both count as an unarmed strike and work like the unarmed strike in basically every way, but the gauntlets are automatically lethal, do not provoke Attacks of Opportunity, and the spiked gauntlet’s base damage is 1d4 (this does not stack with other improvements though, so if you have 1d6 base unarmed damage, all three use 1d6). Improved Unarmed Strike also ups unarmed strike and gauntlet damage to 1d4, so the damage difference rarely comes up.
Unarmed strikes can be “dual-wielded” for Two-Weapon Fighting and you can have at least two versions of it for magically enhancing, as you would with two weapons.
I’m now going to add in that bit about eager et al. because that’s a good point. ;)
No, by a strict reading of the rules, monks are not proficient in Unarmed Strike, but literally everyone houserules that. Whether or not that houseruled proficiency extends to Gauntlets depends a lot on how you answer the question of “Otherwise” above, and exactly how you want your houseruled proficiency to work. Since the actual rules don‘t actually offer that proficiency we cannot judge strict RAW here.