I think that, ideally, the tooltips should read in a way that displays whatever method of AC calculation you are using already, plus the difference between that and 16 attributed to Barkskin - so, an otherwise unarmoured draconic sorcerer, for example, the tooltip might read "AC 16 (Base AC 13, Dexterity +2, Barkskin +1)", and the extremely clumsy druid might have "AC 16 (Base AC 10, Dexterity -3, Barkskin +9)"
This would convey the function that Barkskin is actually fulfilling more clearly, I think.
Yes, I think you're right. Though any change that actually had the numbers given actually adding up to 16 would be an improvement over now, as you say that particular option clearly shows what benefit is being gained from the spell.
