I don’t “get” the troubleshooting. Doesn’t make logical sense.
If you pull the “S” wire there’s no way no how that the solenoid/starter will kick in to test anything.
A short on the “S” wire won’t keep the starter engaged. A short will burn up your harness (and possibly boat, because there’s no over current protection on the harness.)
The procedure would be to start her, then pull the “S” wire (at the solenoid or panel end) and see if the starter kicks out. If so, it’s the start switch that’s fubar, not the solenoid. (Actually It’s safer to disconnect it at the panel and just energize it in order to start. Or disconnect it and use a clip-on remote PB start switch on the “S” terminal.)
It the starter motor stays engaged with the “S” then disconnected, it’s not as easy to tie down. It could be electrical, ie, the solenoid or even the starter itself (the way the Denso starter operates) or it could be mechanical ie, the overriding clutch not disengaging once the flywheel spins faster than the starter pinion gear.
Yes, the solenoid is separate (common knowledge among official IC30A forum members, we run across this **all** the time.) LOL.