[Memnoch]

1) Sorcerers get 3 Lore per level, just like Mages, right?

Correct.

2) Should Monks be grouped in the Warrior superclass or the Priest superclass? The manual says they're Priests--but they can't cast spells and their job is to beat things to death. Whose hitpoints/THAC0 do they get? What about weapon proficiencies?

Warriors - IIRC the game treats them as a warrior "kit" with some special abilities.

3) Exactly what items can Wizard Slayers put in their Quick Item slots?

Healing potions I think...don't quote me on this, it's been a long time since I've played a Wizard Slayer.

4) It seems that when combining two classes (such as Fighter/Druid), the character gets the armor skills of the stronger class (in this case, you have a Druid wearing Plate). But if a kit is involved (for comparison, an Avenger->Fighter), does the character have the armor restrictions of the weaker class (a Fighter that can only wear Leather)?

Your second assumption is correct.

5) Is the Kensai banned from all types of Bracers & Gauntlets, or just those that grant a base AC? What about as a Kensai->Mage?

All types.

6) Can Stalkers Backstab with any melee weapon that a Ranger can use (such as a Warhammer), or are they restricted to the Thief's Backstabbing tools?

Any bladed singlehanded weapon that a thief can use. Some people have claimed to have backstabbed with non-bladed weapons (warhammers, maces etc) but I haven't confirmed this.