5) Using a shield doesn't cost the character anything. Using a weapon costs energy.
Only if we say so. Causing tiredness to the defender is plausible, but by no means necessary. Also, since an attack would now cause damage to the attacker, we wouldn't have to rely on energy to limit the number of attacks anymore and might even get rid of it completely.
6) Being able to adopt a stance or equip a weapon in no way allows for an appropriate response to any situation other than the one anticipated. If my guard character secretly is worried about an incoming galleon and adopts a combat stance, should he be allowed to then slaughter half the town when they decide to have a pillow fight?
Seriously, if it is common knowledge that a guard with a sword in his hand will strike when attacked and it's possible to see whether he's holding a sword or not, and the
whole town still insists on hitting him with a pillow and doesn't even stop after the first person got hurt, they deserve to be slaughtered.
The only case where the mechanism could be abused is when a person wants to commit suicide by attacking a guard, but that's something that happens in real life, too.
edit: Concerning 7, see 5.
Pretty in pink.