I will tell you from experience. I had a similar situation and could not figure anything out. Was on the lake, full boat, had to get towed back to ramp. I was taking things switching leads etc. all things you are trying. I wiggled the throttle a number of times you name it. Finally, was getting so upset that my boat would not start, I put the throttle in WOT, still wont start. put it back in neutral and walked away. Came back a few hours later, tried to start and it started right up, no issues after that either.

Throttle was not fully engaged in neutral! by going to WOT and then back, the throttle engaged properly into neutral.