Did some tinkering today after reading the schematic a bit more and managed to fix the high score reset issue. At first I thought maybe it was a column failure, seeing as the column consists of Plumb-Tilt, Ball-Roll Tilt, Right Coin, Center Coin, Left Coin, Slam Tilt, and High Score Reset, all of which were not working. But, it's also responsible for the Credit Button which IS working. The whole column couldn't be in a fail state.
The next obvious reasoning was a bad wire. All of them run through a Green/Brown wire back to the main board. But since the credit button was working, I surmised that it couldn't have been a failure on the board, but somewhere else. The five connectors as seen in the image above (high score reset, sam tile, left/right/center coins) all exist on the coin door beyond a 12-pin connector jack. All five of these were non-functional. Therefore, the issue is there.
And certainly it was. The green/brown wire that existed to connect all of those mechanisms had been spliced and looped back on itself for some strange reason. I never try to question what previous owners do their machines. None of the five switches were operations. As soon as I re-attached it to the left coin door, they started to work (at least the ones I could test immediately.)
Left coin chute works. Right coin chute does not. But the previous owner once again worked magic and cut the arms off the chute switches. I can still turn the mechanism, but the right one doesn't activate.
High score reset works just fine now! That's why I was doing this.
Tilts I may mess with another time. I believe they were severed too. I just don't feel like taking the machine apart today.
The machine is a beater. I'm not going to replace the coin mechanisms because they're missing and the game it staying in my house. The plumb tilt I will probably fix. But slam tilts and ball roll tilt probably not (if they're missing or broken).