Your transmission should be fine but you never know. The fluids can leak down causing the seals to dry out.
If you think that some of the electricals are unserviceable then get the transmission running and take it to someone who can read and interpret a diagnosis. Don't throw the parts cannon...
Your operating temperature can be above 190 and 195 so what is the use of changing out your thermostats? If you are suspecting that your engine isn't cooling effectively enough then have the radiator looked at. You can take an infrared temp gun and check the the top and bottom of the radiator...
You are correct that the diode prevents a back feed through pin 86. Think of it this way. When the magnetic field collapses at the coil the voltage will instantaneously spike. You don't want that going back to anything like a controller with sensitive electronics.
You can wire up a relay...
Change out your fuel, oil filters and engine coolant on a regular basis and stay on top of your services. Watch the quality of fuel that you fill up with. Keep the engine bay clean and change out oil cooler lines and radiator hoses.
There was a way to make some money on the side when I was in the military. It required you taking a couple of glow plugs and solder them in series on a four foot lead. Call an infantryman over and say "hey watch this!" and demonstrate to him how you could heat up a cup of coffee with it and...
It would be nice if a PCM could do an auto-test on glow plugs. That is not the case because of era involved for GM 6.5. The computer could take a resistance or current reading on all eight GPs give you results.