You can't rest in combat, you have to go to a bed. To heal in combat you need potions or special spells or abilities (which consume action points). You can't rest for a while after resting - that was in Divine Divinity as well to prevent abuse. If you rest up, any enemies who were damaged also heal back to full. That's all you need to prevent abuse.
But that's a really good point about resting in multiplayer. I don't have any solutions on how to fix that other than "both players need to agree before they can rest."
I completely disagree with your idea of locking the skill bar in combat. This game is not using a Vancian magic system where you only have X slots and can't change them until the next day. It is a quick skill bar, you can know more than nine skills. If it's your turn, you should be able to open your skill menu and switch out your quick bar skills as much as you like for no cost. The quick skill bar's purpose is for convenience, not INconvenience.
