WMS built in diag is pretty limited and the board needs to boot up somewhat to use it (ie has to read the software ROM before diag). If the board is not somewhat booting up the diag button may give you erroneous results and that confuses people. Make sure the coin door is open when pushing the diag button or you get a CMOS ram failure diag code.

If that board set is locked up and the PS is good I would investigate the ROM sockets and interconnect. When I used to fix these boards 90% of boards like yours (not corroded, but not booting) would come to life after I put in all new IC sockets and header pins/interconnect. BTW you have a sys 6 version (green rom) of flash. I would ditch the five ROM setup and go to the three ROM setup (Flipper ROMs stay, Game ROM goes on a 2716 and set at IC14).

You can get test ROMs that allow you to boot up only the MPU on the bench. Some of the test ROMs require building extra diag boards. I always used Andre's ROM just because I was familiar with how it ran. I hear Maralb made a good one too, but i have never tried it.

Sounds like it's booting into audit mode which is good.

Open the coin door. Power cycle a couple times rapidly is another trick to see if you get attract mode. If so the mpu is probably working fine.

When in attract mode you can hit the coin door button to get to test mode. Like your blackout.

Don't have to change the roms if it is booting and stable. Just a single game rom vs three game roms is easier if you have to replace ic sockets. If you are booting you don't have to fuss with a test rom either unless you want to.

open the coin door and power cycle rapidly a few times. If it hits attract mode your MPU is booting and is in good shape (sounds like it is).

from there use the coin door button to get into test mode and see whats up.