With the mileage, I wouldn't be surprised if the motor had a few oil leaks. IF you have a blacklight you can do an oil change with Molygen from Liqui Moly, run the car for a few days then use the light to illuminate the UV additive in the oil to help pin point leak locations.
Molygen motor oil - https://www.ecstuning.com/b-liqui_moly-parts/molygen-new-...
The short answer is no, there isn't any leak stopper formula that will stop all the possible leaks. Your car is oozing, which is very normal after 200k miles!
The longer answer is that the leaks could be coming from many systems:
Power steering unit
Leaks can come from seals, loose or degraded hoses. ...
The pump cutoff is completely independent of your car, it's based on air pressure and has nothing to do with your gauge.
What you describe does sound like a bad fuel level sensor rather than the gauge, usually gauges will stick when they break or just go dead. Your gauge is registering some changes, although it doesn't register full and it reports low fuel ...