I agree. this on is a brain twister. Like you I would start with the easiest and cheapest possible
solution, and eliminate progressively from there.
I had one a while back. New engine, had spark, compression, fuel. Wouldn't start. It had me baffled. I don't know why I thought of this, but I took a magnet from a engine I knew ran and checked it with the one on the non running engine. The polarity was reversed. Weirdest thing I have trouble shot so far. Changed out the magnet and it ran like a top.