1. The penis, I believe, becomes extra flaccid after masturbating, especially if you do it a lot. I don’t know why, this is how it’s happened in my experience. I too have lost gains by masturbating a lot, and no, I’m not trying to bash the sexual aspect of it, I just think that extra-flaccidity makes the penis “heal” shorter than before PE. A thing to note is that PE is a technique to deform the cells in your body to your liking, it works because the body has regenerative abilities and those cells are regenerated pass their original stress point (this is why people in some cultures have ears with huge rings in them, and plate lips).
My recommendation is to not masturbate at all after jelqing or otherwise PEing, it, again, in my opinion, will only result in a wasted workout (if you go very flaccid afterwards I mean; you want your penis to be engorged with blood so that it can heal from the stresses put on it, and you want it to heal in a big state, not a small one).
2. It’s probably an optical illusion, although I can see the foreskin allowing you some gains… I wish I had foreskin… I think I could get major gains if I had one.