You paying for half their expenses at her house......is that an agreement your DP made with his ex? Or is it in consent orders? If there is nothing, why is he still paying it if he can't afford it? Makes no sense to me.
Seems if you stop paying this most of your concerns will go away.
If your DP talked with his ex about the activities and came to an agreement of how many he will contribute to per term, it might be better for you. Then anything on top of that she pays for out of her income.
ETA did he get any legal advice before agreeing to this?
We have recently been in a long, involved custody battle and I can't see ever being told to agree to something like that!