i use to work on the Focus door line and I can tell you that there is no difference in the door, jsut the electronics, wiring and panel. a SEL inside door panel will fit correctly with no issues. Cost from a dealer will be a bit high. the best bet is to start looking for a wrecked car in a salvage yard.
They look separable because a door panel is actually several molded pieces sonic welded together. I also worked in a plastic parts plant where we made door panels. Is it possible to replace... yes, is it a lot of work and hard to do at home without proper equipment... yes. like i said jsut find a wreched car and get whatever you want out of it.