What is referred to as parchment in the US cooking world (it is not parchment, it is paper with silicone) is what is most similar to the release paper. I have no idea how many times it can be reused.

Release paper can be used many times. 18 meters might be a lifetime supply for a hobbyist.

The mat board that I use between the platen and the artwork is the same as the board I am mounting to. Typically 2 ply. I use this board for a bunch of prints, until it is so marred that I toss it. I find the board helps distribute the pressure more evenly.

I heat the press before I put the art in, open it and arrange the "sandwich" inside then close it. Once it reaches temperature I give it about 2 minutes, turn off the heat and let it cool a bit in the press. I don't think you are doing anything wrong with this part of the operation, though.