KissakalaA PSU would probably rather fry any other components if it was malfunctioning like that.
GPU also wouldn't have such a wide range of dying symptoms, as most of them would only cause the graphics to be more or less weird, or you wouldn't get visual input at all.
RAM, welp, I also don't think it could have such massive effects on everything you do, but it's a possibility.
I'm fairly sure the problem lies within the motherboard; either the blackout damaging the MoBo, or the SSD messing up the drivers bad (which would be rather weird), or both.
As for confirming the suspicions about a dead MoBo, I don't really know anything else than somehow getting your hands onto a MoBo that supports the CPU you're using, and trying with that.
unstable 12/5v rail doesn't mean it would fry something, it just means unstable, sometimes the psu supplies more stable current with a stable voltage, sometimes it doesn't but that doesn't always fry things, that just make things work bad, and this MIGHT be the case.
i agree on probably not being the video card.
the malfunction of a ram module could cause blue screens, freezes and pc not starting at all
mobo malfunction could cause it, but it could give every kind of problems, literally everything, it's probably the hardest thing to detect, unless you try another mobo ofc.