> I found some twisted connector cables, and a keyboard connection looks pretty sketch to me (both shown in the pictures), Is this normal?
Not sure if the twist is standard but I can see why it may be done. The keyboard connection is perfectly normal and that's the way the cable is mounted in the factory.
> Also found what looked like mold inside part of the machine...
Think of it as a kind of glue to keep the aftertouch strip in place. The strip is very fragile and needs support to stay in place. I highly suggest you move the strip as little as possible as breaking something within one of its conductive layers will be a headache.
> it got stuck on the welcome screen.
Sounds to me like it detected illegal RAM contents and was performing a factory reset. That may take a few seconds.
> I opened it back up to replace the motherboard battery - which also didn't fix it (the battery is held in place with electrical tape now - if that comes loose, will I lose any user data?).
Did you use a new battery? Why is the battery held in place with electrical tape? The battery holder holds it securely just fine. And yes, the RAM is backed by this battery. Battery low or out = RAM contents gone.
> It seems to be working normally now - and I eventually got it to communicate with Studio One, but the welcome screen freeze concerns me, and it seems like posts about that problem show up from time to time. Is that something to worry about? Is there a known cause/permanent fix if it continues?
If it persists with a brand-new battery then I'd strongly suspect the power supply. As power supplies age, their ability to supply peak currents reduces. During boot-up, the Virus draws the most current, so if the power supply can't keep up due to old age, anything can happen - including it getting stuck on the boot up screen. A new power supply can be had for relatively cheap, so if the issue bothers you it could be a good investment. Or maybe you already have a suitable one at home that you can try. Any current supply ability above 1A is fine - just watch out for the voltage and polarity.