Will he stay alone anywhere? I never let Sully out off leash but she will sit, tied to a post for hours alone. I am always in sight, but I let her wait outside while I do laundry, for example, so she can watch the chipmunks and birds. When I first adopted her she would freak when I was out of sight but she got much more confident over time. You don't mention if he is still a puppy but I imagine he will eventually go alone. If you can watch from a window, as I can where I live, you can open the window and give him praise and a treat when you "catch" him going.
sounds like he needs to work on getting used to the dog door. Our Rocky was afraid of the dog door after he got slapped in the face the first time he followed Rosie through the door. It took a while but with the hubby on one side and me on the other, we put his leash on and almost had to pull him through the dog door to get him to use it. He got lots of treats for going through it and he got the idea. Now he charges through with no problem. Corgis will do almost anything for a treat so make going through the dog door worth his while!!