When equipped, Kana's Turban's +1 Intellect suppresses Drinking Horn of Moderation's +2 Intellect. As I understand things, this is the opposite of how it should work. If you have the drinking horn equipped, unequipping Kana's Turban increases your intellect by 1.

Generally speaking it should work in the opposite direction, yes. Highest bonus should prevail. But the Horn is the one with the "only when > 50% Endurance" +2 Intellect, right? It probably goes wrong because it isn't an unconditional bonus, I suspect the stacking system isn't quite able to deal with that properly.