Johnnyloans
Point #4 was something I ruled out. If the power button is held down through a complete reboot, it doesn't subsequently bring the menu up and reboot again afterwards. I tested this by holding it down for several minutes through and after a complete reboot. Granted, I didn't hold it down for 15+ minutes as I didn't think it was necessary beyond a certain point as the reboots happened very quickly.
It was sitting on the desk in front of me, there was nothing that could have depressed the button long enough to cause it to reboot three times other than the phone case. I also tried to wedge the phone in the case to see if I could get it to press the button hard enough to force a reboot. And there's just no way it would do this. It takes too much force and it would have to be sitting in such an awkward position.
The power button seems fine. I also gave it a few days to see if it would reboot again and it hasn't done it since.
Water damage does seems like the most likely culprit here, it's just a little strange it hasn't done it again. With every other phone I've owned, if something breaks like this, it's not a single event affair.
The only other thing I did with my phone that I wouldn't normally do was tether my work computer to it because my internet was out for a few days and I needed to work.
Out of an abundance of caution I flashed the phone back to the stock Pixel OS and re-flashed Graphene onto it again. I'm probably being overly cautious, but if I had seen it do what it did without the potential water damage being a factor, I'd have bought a new phone already.