Okay, so I'm the local resident alchemist lover, always re-reading the Treasure Companion, so I'll go ahead.
1. Yes and no. You cannot give a straight armor bonus to a non-protective item as you would an armor or shield. What you can do, however:
A) Create a general item giving a bonus to adrenal defense. Obviously, that means the bonus only applies where adrenal defense applies, and under the same conditions. For monks though, bandanas, etc. will work wonders, but it might be less useful to those who already wear armor, use weapons, don't have spare action %, etc.
B) You can give an armor bonus to an item that can be used for parry, but the bonus would only apply if actively used to parry. For an exemple, a sword could be given a +X DB bonus similar to an armor, but it would apply only when full OB is used defensively - ie: when the sword becomes a shield. The nice thing is it would stack with the magical bonus of the sword itself.
C) You can create a constant item with a defensive spell on it, from Shield & Blur to Deflections I which would automatically be used 1/round(would require item intelligence to work though), etc.
D) Daily / Charged items, with more limited impacts.
There are a few more ways I could think of that would be more obscure. But the basic way you could get a ring of +X DB would either be through constant item(costly, and requiring lvl 30+ alchemists), or through a general item of +X to Adrenal defense, again limited by the restrictions which apply to adrenal defense.
Another thing I could think of is using an Armor spell on a circlet to make it protect as a helm, the same way you could make a buckler protect as a wall shield for exemple, and it would require a separate enchantment to also give DB. That would be another option.
2: To get AT 3 or 4 from a robe is easy, it only requires an appropriate level armor spell. To get AT 5-20, you would need a constant item with the appropriate spell giving an AT imbedded.
3: Yes, robes are AT 2 armors, they can be enchanted with regular armor spells, or given a constant spell of Enchanted Robes, for exemple, but the former option is cheaper.
4: Depends on the spell used, the level of the caster. I'll direct you to p. 53 of Essence Companion for a good table. Without giving the same numbers, you might want to consider base cost of 2 bp / level of the spell for pure spell users, adjust it down if the spell isn't a base list, adjust it up if the spell is high level. Spells which might have a direct market value (Herb mastery spells improving the effectiveness of an herb for exemple, Earth Law used to build a whole building from scratch, etc.), a risk factor, legal implication, etc. might also have different costs.
5: Actually, there isn't, that's in D&D, and I haven't found any reason to believe keying an item to a class gives a cost reduction, despite my digging in the Treasure Companion repeatedly about the subject. Keying in RM is basically a way to control who has access to the item, not a way to reduce costs. And its highly suggested to DMs not to allow keying to particular individual, mostly for balance considerations (if items could be keyed to work for someone, all old elven relics might end up unusable, as they could be created for immortal King X who doesn't want his toys stolen, for exemple).
