Question: Before taking possession of Deputy Josh, why did Jason shave off the mustache?
Question: How do the hockey mask, clothes and machete appear when Jason is reborn through Diana's body, not to mention how was he was reborn to the exact decomposed state he died in?
Answer: Of course he would be reborn exactly like he was before he was blown apart. Which is why he needed a blood relative to do so.
Chosen answer: Evil magic.
Question: At the end of the movie, after Steven tackles Jason in order to save Jessica, Jason starts brutalizing Steven. Why didn't Jason simply kill him right away? He could have quickly killed Steven in various ways, like crushing his head, punching through his body, snapping his neck etc. Why was he brutalizing him instead of trying to kill him as soon as possible?
Answer: The honest answer is "because it's better for the movie/narrative that he doesn't try to kill him right away." But I suppose you could also argue that Jason is just toying with him and prolonging his death out of fury.
Answer: There isn't much of a feasible in-movie explanation. In truth, the entire scene was added by the director because he felt the movies were pretty sexist in that it was mostly women who were either nude or sexualized. Thus, he added an inherently homo-erotic sequence involving one man shaving another stripped man to "level the playing field" so-to-speak. The only real explanation I could think of is that maybe the creature just thought it'd feel uncomfortable transferring to a man with a mustache. But even that's shaky at best.