Klorox wrote:1) does this apply to non-thief weapons (if the character dual classes)?
Yes, and missile weapons too. It can be very effective with Melf's Minute Meteors or similar fast-attacking abilities.
Klorox wrote:2) is it applied at levels 1, 5, 10, ect. Or is it applied at 1, 6, 11, ect.?
5, 10, etc. Their THAC0 doesn't quite catch up to a fighter's, but you could always dual-class (I like to switch at lvl 25). Note that Swashbucklers don't get the extra 1/2 attack from weapon specialization, so darts are quite nice, and in BG2 there are a few weapon options that add extra attacks.
Klorox wrote:3) does the AC bonus apply in non-thief armors?
Yes. It also stacks with potions of defense, making for quite the impressive AC no matter what you're wearing.