So, having repaired the first broken Mercury II and it seeming to have held I've now done the second one.
This was exactly the same process as the first repair. The second and third photos show how little wood there is in the bridge area, it's broken through where I've drilled the holes for the bushes out.
It's not like these are huge bushes either, I was drilling a 10mm hole. While this is the 'large' size bush I've noticed in use it's just that, not super-extra-humungeous.
Once this was done, it was just a case of refit the neck, solder the electrics back up to the outlet and earth then set it up.
Sadly this one is probably going to need me to shim the neck. The 'lockdown bar' reduces the clearance between the tremolo base plate and guitar body and with even a slightly high action it was close enough to restrict movement.
Once together it plays nicely enough and I think I prefer the neck to the 'padauk' one even if they're ostensibly the same guitar.
None of this answers the question about what I'm going to do with these long term. I think I'm going to shed a few guitars and these might be going straight back out of the door.