RE: 2002 bad fuel filter?
I'd still keep water in the tank as a possibility, although your next step after STP is to totally drain the tank and fill with fresh fuel.
Just in case you didn’t understand what STP does, it allows the water to mix with the fuel, rather than separate from it. That way it gets out of the system buy being burnt in the engine. You could still have a lot H2O, but now it is suspended at ALL levels of the tank.
Im kinda thinking out loud here cause I’m not in your shoes, nor am I near the car to see and feel how it reacts. But if adding STP made the ¼ tank problem become an ALL tank problem, I wonder if we didn’t hit the nail on the head, and you had some REALLY bad water and gas issues?
As for plugs, don’t waste time or money on a lot on fancy stuff. Simple ND or NGK will work perfectly.