I know this is an older post, but it deserves attention.
This is an excellent question. I run games using the same assumption that weapons can never go above a "modified" +10 enhancement bonus, so it's nice that we don't need to debate that.
My answer to 1) 2) and 3) is yes. It would be incredibly odd to me if the Warpriest and Paladin abilities limited a weapon since the whole idea is that these classes have a powerful magic mastery over that particular weapon.
