Wow, that's quite the list of improvements! I'm not familiar with the layout of your trailer and I've not done any of these myself. I think I remember reading a thread here where someone did add a door to make a pass through. For the cushion foam you might try a local upholstery place, they can give you some advice on the best type of foam to use and probably cut the pieces to size for you. For your shower door, Camping World sells a pleated door that runs in a track which might fit your need.
Otherwise someplace like Home Depot might have something workable. Let us know how you make out!