It was a tough job, but I finally got it all done! I thought I was six inches short, but after fitting in the new elbows I needed, I was about a foot too long! Moby bogosity!
In the end, with no new pipe that I could lay my hands on today, I was forced to that last resort of builders and installers: field-fabrication. With great trepidation, I measured as well as I could (multiple times), then cut down the overhead run (the one at the angle) and fitted it into the elbow. A little WD-40 and a lot of elbow grease, and everything fitted together. I was worried that I was a little high on the end piece running into the wall thimble, but everything works just fine.
I have been wrestling with this iron alligator for over six months. Sweet Polly Purebred! am I glad to have this done. Next time I go out, I've got to clean the cabin and make it ready for guests, but at least I can go stay the night and be warm, instead of going back and forth in a single day. Working on this thing has been like working in deep water: you spend half the day getting in position, only to make three 15-minute dives, which is all that's safe to do. I'd drive 3 hours to work 3-4 hours, whereupon I'd be too shaky to safely operate power tools or climb ladders -- then run errands and drive home.
But it's done, and it's right. Hallelujah!