Okay, so I know I am not a copyright law lawyer or anything else, but given the questionableness on how legal fanfic is, how is all this remake stuff (which seems more blatantly copied than most fanfic) remotely legal?

Because they'll be buying the remake rights from the copyright holders (AFAIK, remake rights usually belong to one of the parent media corporations involved in funding the show, so the show's actual creators get no say in it, but hey ...).