I have a slightly different take on this. These sellers are difficult to work with, which in the long run reduces the value they will get for their property. The property has dropped out of escrow before, which I would attribute to them being idiots and making hard-line demands which scare away buyers like you. It seems to me that they think they are drawing a hard line and negotiating. But, they are just going to lose another buyer and further taint their property. TO get a max price and gain form a home sale the seller needs to be accommodating and open, at least that was my approach in the past when selling and I think we extracted max profit when selling.
Use this to your advantage. Be willing to walk away, and don’t assume that their take-it-or leave-it caps on closing costs are set in stone.
Once you get an appraisal, ask for whatever the hell you want. If they have demands that don’t line up with your needs, then move on. Their loss.