John Cribbin

Looks like the various parts are connected up with plugs/sockets. I'd be tempted to unplug and replug them, if you're lucky it may just be a poor connection or dirt/corrosion on the pins. I doubt if that fuse is for the power amp section, but just in case have you checked it? Obviously do this with the unit disconnected from the mains ....

First thing I'd try is to clean all the sockets. Dirt / corrosion can cause those types of issue.

I'll second PlusGas, it's specifically designed to do the job. On cars I've had WD40 not release nuts after 24 hours soaking. Plus gas half hour normally and the jobs done. Personally I'd get something like a thin screwdriver, spray some fluid in a jar and use the screwdriver as a dropper to get the fluid just where you want it.

Surely if you defret and don't fill you're effectively creating a kerfed neck which will be more prone to a forward bow under tension?

As it's Japanese, there's a good chance they've used JIS screws.

I'd try Kent Armstrong, they have been doing custom stuff forever.

I'd guess box spanner, they have pretty thin walls.

For years, I've protected new cables and cabs with Armor All Protactant, you can get it from most car accessory stores. For old stuff. I wipe down with warm soapy water and use naptha for any sticky crud. Then I apply the Armor All. About once a year, I'll spray some on a kitchen roll and pull the cables through a few times, removes the old dirt and reapplies the protection.

You need agedhorse who is one of the guru's at Mesa Boogie and is very active helping with MB issues. I don't know if he is still active here, but I'm probably breaking a forum rule by suggesting if he doesn't pick up here, post the issue on Talkbass were he's certainly active. Good luck with getting your issue resolved.

Good luck for the op and a speedy recovery. OK here's the tough love. Don't ask a bunch of randoms for medical advice, it won't end well. You follow doctor / therapists advice. If recovery is slow, so be it. If you rush back before your body has recovered, you may cause more damage than you had pre op. Take care and be patient it will be worth it in the long run.

Years ago, I had similar with an Ibanez. I bought a cheap set from eBay to canabilise for spares. Something like this: https://www.ebay.co.uk/itm/134092048521?hash=item1f38824889:g:KjYAAOSwa-diX-Mo&amdata=enc%3AAQAHAAAA4BCk6H0dxo6Ge0VRB4He2isMz2NKWGvuUsoqocGBuJqIDBZ8M7RCj0XWkaykrhv7VnqxKnl3O%2B1gjqJYcblidgrn3p1UXBtgEYq457mznELCkblOfgzHX5LJP6rZQxrryEjRT9wSua%2Bpy73GrKIZrjVe7uDo5VVG7ntJmZdWPS2z49q4dxJu3buNallP%2FfyoCFyAdWxbLr%2FUx%2FKh3pXmqYbkPI%2BZKUYox4i%2BoqdW4bSoEXMiJx1CdSO16bbbJSAVSZePcINTQhqW9eiYcCkz%2BO%2FuEU6GfjGOXJ%2BLadYcyvhN|tkp%3ABk9SR6yJpenhYQ

You could used another sticker on the front, job done!

Depending on age, Peavey may be able to help. Other than that, you may be forced to keep looking at eBay until somebody offers a used part. If the demand becomes high enough, at some point, somebody will create a 3D printed part. In the meantime, I'd suggest velcro to keep the battery in place.

Fourth one down? https://www.sandberg-guitars.de/support/wiring-diagrams/

It's a bit difficult to work out without the above details but .... Working on the assumption that the loose wire has not strayed far from it's home, the bottom right connector just below the loose wire in the first two photos, does not have anything connected to it.