Seam cracks, how to fix them so they don’t crack again?

1)They may have finished the joints without taping them first. You may have to go back and tape them to prevent cracks. If the mud isn’t on too thick you could try taping over what is there and feathering the joints out about 12 to 14 inches on each side. If you are good at mudding, you could put a skim coat over everything for a smooth finish.
Once a joint is “finished” without tape, whatever you do is going to be a fair amount of work.. I really don’t know of a fix without using tape. Good luck.
2)This is a never ending battle. I unfortunately would say you will fight. I do have one alternative that might work. If it happens again there is a product called elasto patch which is a drywall mud that has flex. Using this product would allow for some movement without cracking. This may be a solution verse redoing the drywall. I would also recommend adding a few additional screws to try and limit the movement. I hope this helps.