Too Much Mud Leveling over New Drywall?
First, it shouldn't crack. If it does your putting to much on at once. Id also stop using all purpose mud after your first coat and switch over to plus 3 to make sanding easier on your body. I definitely wouldn't scrap off the mud you have already put on. If you want to sand some mud off Id probably get a finish sander and sand some off if necessary. But go easy on the sander. You can screw things up real quick if you go crazy. A pro would never do this but then again a pro wouldn't make your mistake either. Skim coat with plus 3 a few times, sand it and don't worry about it being two perfect. A wall is never perfectly flat and the only guy that's going to notice it not being perfectly flat is you. As far as primer goes I'd ask the guys at wherever your buying your paint. Normal primer probably will be fine but I'd go with drywall primer.