Would this problem be solved simply by setting it up so that you need to use, say, three full bars of stamina before you can get your portion of the skill points for that battle? Not saying that the amount of skill points provided should change, just that you would need to 'prove your involvement' and rather than having it set up 'per action' (which can lead to people using small-stamina actions too often rather than being tactical) you can set it up as 'per overall stamina used in battle' which would be much more sensible. Auto-attack could cause a problem with this, but then, auto-attack would mean that they are involved in the battle and they would have every right to gain the skill points even if they are AFK once they begin the auto-attack. Having said that, even if auto-attack did come in but did not use stamina, then people would need to continue using manual actions in order to gain skill points. Good or bad thing? Well, that's a matter of debate, I imagine. I've no idea, personally.

I'm sorry if this (stamina use = skill points) has already been mentioned as I searched this thread for 'stamina' and got not results. I have not read every post.