Great game Volcano, I like it more than my Haunted House
Sorry, my machine is in storage so can’t help much with pictures.
I don’t have a manual, but this should be the switch matrix numbering:
http://www.pinitech.com/switch_database.php?name=Gottlieb_Volcano
Switch test being #18 should display 99 for all open.
I would be checking switches #20 Outhole, #30 & #40 trough for the drain issue and remove the apron to better see what’s going on. Just in case, Volcano is a three ball game
For the skeeball issue, switches 4, 14, 24, 34 (Craters) & 00, 10 (subway). Maybe #46 under playfield slam switch is getting triggered with the ball exiting the subway? You might also want to meter test all playfield switch issues back to the boards, remembering that there could be some large flaky connectors along the way.
Ground mods are a must and most recommend (I’m still learning) redoing all the edge connectors at the same time to eliminate ‘haunting’ issues later on. I just switched on my Haunted House a couple of weeks ago and some of the playfield controlled lamps were stuck on. Turned off machine (always do this first) and re-seated all the connectors and all good again, which reminds me I need to order one of these kits before they run out of stock.
https://ksarcade.net/gottlieb-system-80-a1-a3-harness-rebuild-kit.html
You stated you had a new MPU, what brand Swemmer/Pascal? Might help others give you some better tips, as I’m no Gottlieb expert.
Good luck.