I don't have any experience with composite molding if that is what you have or manufactured homes. You having such a big attic may be a clue though. That means a lot of open space in your rafters that may allow for some vertical movement without the webbing from rafter to joist. It could also be in your foundation. With a wood floor, if you don't have the appropriate girders and/or piers in the right places it could produce stress along certain stretches of wall. The cracks developing in your sheetrock could be a sign of that. All of that being said. The way I have fixed this problem in the past is by taking down the crown molding. I have only worked with wood crown molding. If you are carful you can do it without damaging it and the wall or just purchase new crown molding. When you put the molding back up you nail and caulk it to the ceiling only. Do not nail or caulk it to the wall. This will allow the molding to be kinda free flouting. Whether its your ceiling going up and down or your floor going up and down this will fix either. The key to making it look good is a tight fit of the molding against the wall. Because you are not going to nail or caulk the bottom against the wall you don't want to see any real gap. In the future if you feel the house has stopped moving you can always go back and nail and caulk the molding to the wall. Your gap at the joints, I don't know. If you don't see any separation of sheetrock in the corners I guess it might be the composite contracting.