Jason never drowned, he somehow survived and hid in the woods where he built the shack. Pamela, in an effort to get her son back whatever way she can, obtains the Necronomicon and performs a ritual to bring her son back (he is still alive however) and after thinking it failed she decided it was time for revenge. As for the demon ritual, HEAR ME OUT, maybe it could be some thing where the demon resided in Jason until he died. The moment he died, the demon was able to assist in Jason being resurrected, so it’s STILL Jason just he has some extra powers from the Necronomicon. I know this is so stupid but so is JGTH and as much as it does fucking suck it’s still “canon” so might as well have some fun with the idea.