Sunseeker 56 Predator stbd engine won’t crank

MM, there's no engine management system on your engines, really.
The wired black box that you are likely to have on an e/r bulkhead is only a concentrator for routing the probes signals to the instruments, and nothing else. You could even throw it overboard, and the engines will continue to run - just with no working instruments.

So, even if I understand what your mechanic said about starters never going bad (having never heard of such occurrence myself), it does sound like it happened to yours.
Anyhow, again, since the engines are 100% mechanical, also for your fire system the one and only way to turn them off is by exciting the stop solenoid.
Which is something you can also check visually, because it has a pin which is kept pressed inward when the solenoid is excited (=engine off), while an internal spring pops it out automatically when the solenoid is not excited (=engine on). And when it's out, you can also press it manually.
It isn't very easy to reach it, because it's directly connected to the HP fuel pump, right inside the two cylinder banks, but your MAN mechanic can surely show you exactly where to look.
Which is important, because if by chance the spring that pulls the solenoid out when not excited is a bit worn out, it can stay pressed as if it were excited, and you'd never be able to start the engine again after turning it off.
But mind, this is just an aside, because the stop solenoid could be the culprit only if the starter would crank and the engine wouldn't run.
If the starter doesn't crank and you are sure it's getting a solid 28V supply, I really struggle to think of any reason aside from a blown starter motor.

Starter solenoid should be fairly easy to diagnose provided you can access it. Did you try jumping the solenoid with a screwdriver or similar?

Glow plug? No such thing to be found, on your engines!