That's a blast from the past! I cut my teeth on an Apple //e so maybe I can provide some insight. Typically on startup, the Apple 2 series would display a bunch of junk on the screen and then clear it, with Apple ][ or Apple //e at the top of the screen. It would then boot Dos 3,.3 or ProDOS from whatever floppy drive you had. IIRC, the transition between screen garbage and the black Apple .//e screen happened when the system loaded and ran the system ROM. So my guess is that there is something wrong with the ROM. You are probably right about moisture.
It may be possible to repair. I would take a look at references from KansasFest. It is meeting of Apple II enthusiasts that discusses stuff like restoring and repairing old Apple II systems, among other things. For example: https://www.youtube.com/watch?v=MFfjdIWe...
"So You Want To Repair Your Apple II — Jay Graham — KansasFest 2017"
You may be able to connect with some of these folks. Jay Graham appears to be The Man when it comes to repairing/restoring Apple II systems. His twitter handle is @jwg1962
That community may also be good sources for the technical manuals and other info. I would start here: https://a2central.com/category/help/
I hope this sends you in the right direction.