Ok, so e-liquid does contain water, so we are dealing with water damage. It would be worth getting some pictures if possible, specifically around the LCD area, and the game-card board and it’s connectors.
I would probably start by having another go at it with the IPA and a toothbrush. Concentrate on the LCD connector, game-card connector and the headphone jack area.
Do you have a multimeter? If another round of cleaning doesn’t help you will likely need to make some measurements around the board to work out what is wrong with the LCD.
Worst comes to worst though, the headphone is on a separate board that can be swapped out in it’s entirety.