If your going down the thinners route, I'd try white spirit first, its less likely to cause damage, although also less likely to remove the paint !
If the warm water has removed some paint, maybe you should carry on that way, may take a while but could be the safest ?
I've got some "knackered" black leather p6 seats that I won't be using, if you want me to test some thinners on one I will.