Hi,

In brief (also w/out direct exp with that particular transistor)

1) those both look like N channel transistors, so you should be able to use the 540 given that you want to put in a 10ohm coil as replacement (at 50v, 5a draw and as you stated the 540 is good for 26a). I didn’t completely compare the rest of the specs but it sounds like that shouldn’t be a problem.

2) just put a probe on the back metal tab, and the other one on each of the three legs one at a time. The back tab should be connected to only one leg. If it’s tied to two Or three legs, the transistor is fried.

3) not a bad idea, but you should be able to replace stuff and just see if it locks on again on power up or game start. If it does, immediately shut down Game and look for other issues (could just desolder a lug while you do this)

4) yeah I’d go for it.
5) sound idea, should work esp. if you vice the coil before pounding on dowel.

Good luck!