I'm an electrical engineer by training (although I'm not a practicing PE), and I just wanted to chime in here:
- It does look like there was some water/liquid present in there at some point. From the photos it looks pretty old, but that's a really difficult thing to prove (read on, because that's not really your problem).
- Liquid around the screws isn't going to break the electronics, but are there signs of gunk/residue anywhere in/around that multipin connector in your second photo? I can't tell if those black spots are printed on the purple wire (and a few others) or if they're mold/residue.
- Regardless, that connector is likely where the evidence would really be-- shine a flashlight into the empty pin slots and see if there's gunk in there. If there is, yeah something got in there and that's just asking for a short.
I'm also a veteran of the customer service world and I'll give you this advice: if you know you're not at fault, stop doing detective work. It is not your job to prove the negative case here and frankly the more knowledgeable you act about the problem and the more you try to prove you're not at fault for something like this decisively through physical proof, the more they're just going to bully you.
Instead, you need to move this from a mechanical/electrical discussion to a customer service one (as DFrantz says above, the service guys aren't likely acting in bad faith here).
You should establish that it was a failure of the CPO certification to have sold you a car a few months ago with water damage. Stand steadfastly by that position and do not waiver.
"I was sold a car that had water damage and you missed it, certified the car, sold it to me, and now it's broken" is probably the line you need to stick to (since if you're tellling the truth about not being the culprit, this is entirely true). Escalate if you have to "is there other water damage to the car? Should this have been a salvage title?"
My point here isn't to be a jerk, it's just that right now you're having a discussion of "can I prove I didn't damage this" and you need to be having the discussion of "can they prove they didn't sell me a water-damaged car" because that evidence of water damage is just as damning to the Volvo CPO certification failure as it is to you as an owner.
Just my 2c. Hope this is helpful & good luck!