Tower shield should still give full cover.
And with the facing thing, you wouldn't pick facing. You would, for example, with a large shield (+2 AC) declare two opponents on your turn that you were using the shield against. You would get the cover bonus against everybody, but be immune to attacks of opportunity from those two.
I agree facing is a bad idea.
Facing = double rogue flanking screwjob for people holding shields.
I would even say that magical enchantments add to the number of people the shield blocks. That's how I've always seen a magical shield, as something that moves itself to block (no need for an explanation of how, or it's not magic anymore), as opposed to armor, which is just hard.
After all, if a shield's AC bonus had anything to do with hardness, why do wooden shields work as well as metal ones? And wouldn't one successful shot pretty much sunder the shield?
Thus, I wouldn't give bonuses for adamantine shields.