Oh wow, seems like you encountered all the possible problems! Good thing you had two C64 so you could swap components. Do you have an idea why so many things had failed? Bad power supply blowing up chips maybe?
There is a voltage regulator in the standard Commodore 64 power supply that fails catastrophically. Specifically, it sends > 5 volts down the 5 volt line.
When this happens, depending on how long it's on it can fry every chip on that line.
That's why everyone advises not to use the original power supplies even for a quick test. Even if it tests 5 volts before you plug it into the computer it can fail at any moment.
Back at the time, we (little me and my family) already knew it was bad. But we weren't aware of its failure modes. All we observed when measured was low voltage, and that it "sometimes worked".
That's why so many C64s were damaged, and still are, when people connect their old supply without the knowledge (or against warnings) of such a nasty design flaw.
When this happens, depending on how long it's on it can fry every chip on that line.
That's why everyone advises not to use the original power supplies even for a quick test. Even if it tests 5 volts before you plug it into the computer it can fail at any moment.