OK the way I interpreting this there is no flipper relay on Sys9. The relay controls the path to ground for all special coils and the flippers. The transistor that drives the relay is Q86 2N4401 and it doesn't need anything but the pre-driver because the relay itself takes the place of the TIP122.

Test Q86 and also look at the solder joints on 1J19 and the relay itself
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keep in mind the grounding test doesn't tell you the transistor is good. It only tells you the path to the coil is good. Test the 2N4401 in diode test can compare it to a known good working one

I think Andrew is on the right track.

Question. Do the flippers work when shorting Q86 and activating the relay?

so heat up failures are usually related to loose sockets. Turn the game on, play a few games then open the backbox. Try and twist the socketed chips or pull them out with your fingers. If you are manipulating a chip and the game locks you should replace that socket.

It would be worth the wait to order more sockets