What ever happens, please update the thread with results, there are many threads without a final result.

The tanks use a venturi pump fed by return fuel to move fuel from the left sump to the right, it is a pretty wanky system. Take a look how a mid depth well water pump works and you will get the idea how it works. ( This would be the kind that sits in the basement and the venturi is dropped into the well, not the pump / motor type that is dropped down the well. )

A transfer tube below the drive shaft would have been better in addition to the existing transfer above the drive shaft.

The actual venturi pump has no moving parts so there isn't anything to wear out. There is a chance that someone replaced the fuel senders ( the wiper corrodes giving an inaccurate reading. ) but didn't rehook the piping properly. If the fuel filter is clogged / pump failing to the point there isn't enough return fuel volume, it could leave the right side fuel pickup dry.

Not sure why you get hard fuel spray when the cap is removed. From what I recall there "might" be a expansion tank in the filer area and I'm wondering if someone got creative with replumbing the system. There is a 1/2" vent and two 5/16" breather hoses in the area of the fill neck.

These cars use 2 fuel senders to calculate fuel, pull the covers on left and right of the tank to access. I'd ohm out the senders to get an estimate on how much fuel is on each side. Rather than running the engine, I'd bridge the fuel pump relay ( in the trunk ) and pump fuel into a can. ( don't catch fire )

The tank on these can't be removed unless you pull the rear sub frame. The senders can be removed buy you need to make a tool to turn the locking ring. Make sure you drain fuel from both sides, the senders are on the bottom of the tank and all the fuel will fallout.