Start with the easy fix. Check your fuses to make sure they're good and check to make sure the fuse holders are holding the fuses tight. The flickering with a pop bumper activating is suspicious of this.

I'm going to warn you up front it has been many years since I had my head inside an EM. But, as a recall a couple of things you could check would be the fuse holders. Depending on what kind they are they can get loose or corroded. The other thing it the tilt relay. The lights will all run through a switch on the tilt relay so when it is thrown they turn off. Might be a switch that needs cleaned or is just out of adjustment and barely making. Most of the work I did was on Bingo Pinball machines. So, I'm assuming that a flipper pin has a tilt relay as well.

Check that all your connectors to the backbox are firmly seated in their sockets and clean.

I was going to say check fuses as well. My Sweet Hearts started to do this and I replaced the fuse and holder and that sorted it. Must say that they looked ok and didn't seem to be the issue but it was.

Even though connectors/fuses seem ok, give them a press/wiggle and see if you can force them to flicker/come on with gentle pressure at fuse/clips or connectors.

If not, check your schematics and find where ALL the backbox lights get their power and start poking around with a meter, following the path.

If you look on your schematic, on the left side you should see a grouping lights called "Name & Scene Lites" with a bunch of bulbs shown in parallel. That's the general lighting for the backbox. The path forms a basic loop from the transformer, through a (typically) 10A fuse, through those lights, and back. So the only thing in the path is the fuse and a Jones plug.

However, right above or below it, you should also see the "Coin Chute" lights in parallel with them. These are on the same circuit (and fuse), but don't go through the Jones plug (since they're not in the backbox).

If your coin chute lights are working, then that indicates there's power on the circuit and fuse is working OK. If so, then the problem would be with the Jones plug, or the wiring in the backbox. But make sure the coin chute bulbs are good and the sockets clean before concluding they're not working (if they're not).

It's a handy debug/isolation indicator.

If you can't get the coin chute bulb to come on, then I'd revisit the fuse holder. It's amazing how just a very little bit of wiggle room of the fuse in the holder will open up that circuit. You can also try gently wiggling the fuse there and see if the lights come on.

With the game off and the fuse removed, you can squeeze the tabs together a bit more to get a tighter fit.

Corrosion inside the clip could be the culprit as well. Even though the clip is tight, if there's a scrim of corrosion there it can insulate the clip from the fuse's end cap.

Sometimes you can have a fuse itself go intermittent internally. When I suspect that, I simply replace the fuse with a new one.