I've done a lot of seats from a mold I made from plaster covered with glass/epoxy and polished smooth.

To do a mold release properly, the mold has to be highly polished - more smooth than a mirror or it will stick. I've lent my mold to people and it always comes back damaged. People have tried mold release (3 coats) vasoline, wax paper etc.

What I do now is is use seran wrap. I can get almost wrinkle free - maybe a few wrinkles. The seran wrap sticks to the seat but comes right off the mold. I use a propane torch and melt it off the seat in seconds and then I sand and coat the inside of the seat. I too have tried everything and had a few times I had to pry the seat off the mold with a lot of force. the epoxy/glass shrinks so it can really tighten up on the mold. I have 100% luck with the seran wrap and I pull the seat off long before it gets rock hard.