Did you unplug ALL switch connectors (205, 260, 207, 208, 298, 212) from the WPC board before testing by jumpering?

All ASIC chips are equal. WPC89-95s. The fliptronic boards may be different, but that's a different board....

Forget anything downstream, opto boards and such....

Get into switch edge test, remove all 6 connectors from the board and test by jumpering 1 row to one column on 207 and 209
(the test is NOT valid if the other connectors are NOT empty)

Did the board test out stand alone before being returned to the machine and after changing out U18, U19, U20?

The F115 error doesn't matter, as long as 205 is plugged in you can get to diagnostics. If you Don't get the error, then the board is not isolated to test.

Once in diagnostics and switch edge test, go ahead and remove 205 as well... This will isolate the entire board from all switches and wiring. You're basically bench testing at this point, with the DMD and machines power supply.

With the symptoms your describing, I would have went for U20 as the issue, but since there still there after replacement of U20, the board needs to be fully isolated to be sure where the problem is... do you have a pic of the board? Was/Is there battery corrosion damage to the resistor network between U18-U20 and the 205-212 connectors?

If you isolate it fully and it still reports multiple switches, then it's time to chart exactly what happens on each column/row when connected, and decide if U20 is still bad, or there is a short in the resistor network going up to U20.

My guess is back to the opto board though, as it was still in circuit if you did not disconnect all (up to) 6 of the connections at the bottom of the board before testing....

And yes, the instructions are not perfect in the fact they don't say remove everything after getting into switch test.
from his note above the procedure: Note: J206 and J207 (column drives) are electrically the same and the connector may go to either one. The same is true of the two switch row connectors, J208 and J209.

That board really took some damage to the back side from the batteries... There's a short somewhere where the corrosion has eaten through the traces, and is still there causing a bridge between them. I wouldn't have said it was that bad from the front side pic, but the back side looks pretty bad.

And for the most part it is now isolated to the board. But if you had fixed it when you were repairing it (something we don't know) and replacing u20, and now u20 is blown again, there could still be a wiring problem in the machine that may blow another board....