You gotta get the flywheel off first and confirm if it is the key sheared or not. If it is, it's likely the half moon key... those aren't hard to find. The you have have to make sure you get that nut tight, and I mean tight!!!
That sooty backfire is a pretty good clue that timing is off.
If you have the engine out, then getting the flywheel off is not much farther. It's best if you have a puller, and if the flywheel has threaded holes, then it's pretty easy to get the flywheel off. If not.... well, a really big hammer, and that's important that it is a heavy hammer, a block of wood, and pull on it by hand while you hit the end of the snout with the hammer and the wood protecting it. Do NOT use a pry bar... bad things happen then as you likely know!
Good luck, I know it's frustrating, but you've ruled out most of the other items.