Understand. I only mentioned it because I had what sounds like a similar problem with a small tuck diesel. There was a fault in the starter motor which killed the cracking capacity of the battery. Went through three batteries on warranty before I found the problem.
That was unlike the outboard which developed a starting problem after I changed a solenoid. In this case I crushed a wire when I fitted the replacement which compounded the situation. Two problems not one. Ultimately I had to re-glue one of the armature magnets to the starter motor body In the outboard [Loctite 326 and Loctie 7649]. Even a mechanic couldn't find the crushed wire - took me forever.
I'm guessing the SM in the mower is like an outboard motor? Magnets rather than armature windings. I also think if you hadn't 'touched' any wiring or replaced any parts prior to this problem, then vibration is the root cause.
Good luck; Is this the first of three problems? They come in 3's you know.
