The bolt not opening after last shot is a mismatch between short stroke JP system and the bolt of the Bul. JP solved this by adjusting the bolt in their JP so you can have short stroke and LRBO. If you keep the JP system and shoot factory change the buffer into some tungsten instead of steel, it slows the recoil (lets say 3 tungsten, 2 steel minimum). If it were my gun i would not bother and install blitzkrieg buffer, cheaper and much better then the Jp buffer. (much much better, and also dead silent). As for the stock, try the Ace stock (fixed one) most longer shooters love that one. You will need a space in the buffer tube since it is originally meant for a rifle instead of PCC (you van buy anywhere ask Sven 3gsports... it's even on their website).
Since you are from Belgium: we will meet at a match soon! BR Henk