Okay, these are interesting behaviors, but the fact that a completely different GFCI also detected a ground fault makes me concerned. I think you should either a) replace the fixture or b) call in an electrician, or c) both. There is something about that light fixture that seems to be developing a ground fault. Even if it is a slight one, this is 120V that could be transmitted to the pool water. I think we've done all we can do. Your panel is in much cleaner shape now, so that's a good thing. You can also tell the electrician what you found, and that will take up less of his time. But I think ultimately there is a slight short somewhere in that light fixture or in its cord. I hope that the conduit between the light fixture and the PLJB is intact. If you pull harder on that cord, you should be able to get some movement unless the conduit is rusted out and crushing against the cord.
So, I'd say it's close but the rule is that if the GFCI breaker goes off, you need to repair the circuit. I would disconnect the circuit from the panel as was done before until you can get a replacement fixture. I'm sorry to be the bearer of bad news, but I think it's better to be safe than sorry.
