Any suggestions for repairs of nasty cracks in my walls and the door?
On the wall you need to cut out the entire square of drywall out, mud and tape it, sand it, paint it.  With fast drying mud and a fan you can do this in 6 hours easy and maybe 30-45 mins of actual work.  

For a door the only thing you can do is try to Bondo it but it should be replaced.  If you are leaving the place they might charge you for the door even if you fix it.  If you are renting it out the Bondo should be OK.