Williams doesn't do any diagnostics at boot up. The leds start on, and as soon as the CPU starts running it turns them off (like the initial flicker on ballys). Any number of issues can stop it from actually booting fully and getting to attract mode, or it could just be something with your displays.

Two flashes when pressing the button means the self test passed. It still misses a lot of issues though, but a good sign.

Leon's is a great beginner rom to just make sure all the inputs are working and the board boots. Andre's is very useful once you know how to use it, but it's hard to learn how to use it on a non-working board sadly.

If it doesn't boot the displays definitely won't work.

I'd get a copy of leon's and andre's. start with leons, make sure you can get it flashing, then use andre's for more detailed testing as needed.

The boards are compatible, but the parts won't be quite the same. I'd just find schematics for whatever version board you have and use them

Instructions and rom images are available online. https://pinside.com/pinball/forum/topic/wms-sys-3-7-andre-test-rom-usage-video-v2

If you're getting digits on the display it sounds like it may be booting then. What are they outputting? It may just be the dead battery/audits reset version number, in which case putting in some batteries and scrolling through the audits should allow you to boot into attract mode

That fuse wouldn't affect anything. You should only get the one flash for a successful boot. Did you try the turning off and on quickly?

Might as well install a remote battery holder now anyway.

They shouldn't...