The steamer it does help! So now the paper does come off, be it still while putting up resistance. I have no idea if I will manage to remove the paper and do the PVA-ing before my bag of plaster will have turned into a large plaster rock, but one way to find out! And in the worst case, the plaster lump will have to go to the recycling centre and I'll have to buy a new bag. But then I won't do that by roadbike!
One small panel stripped
A bit more stripped