I’m hoping to find one over looked item
I'm sure you check it already but I searched your posts and did not see any mention of just taking out the air filter completely.
Also, I think it would be safe enough, please check though, to disconnect the engine from the muffler to make sure there is no backpressure.
Also, the injector elbow has been known to be tricky. I heard one story where the guy boiled his out, the engine would not start so he replaced it and it started. It was something about the inside was hard to see and it wasn't really clear inside even though he thought it was.
If the timing is off I would think the compression check would tell you that.
The last thing you want to hear about that dam shutoff lever. But humor me and disconnect it and the throttle cable.
It may look like it has full throw but who knows. I'm thinking that your test with the injector out of the engine spraying fuel probably tested that but maybe the spray pattern was a little weak and not visible, I'm just thinking out loud here.
The throttle also. The engine may need, at least after all this fooling around to have the throttle wide open to start.
I must be insane because I have seen things on the fifth try that I just didn't see before.
What I'm afraid of in this kind of situation is that I might find the original problem and fix it unknowingly but introduce another problem so I still can't get it to work.
If it ends up being something dumb please let us know anyway as we have all been there. We learn from others mistakes.