After increasingly futile attempts to get the Mac to do anything I've reluctantly come to the conclusion that I buggered something when I "upgraded" it. My guess is it's probably one of the memory sticks, which is malfunctioning just often enough to foul up intallation and gradually getting worse, but it could be anything. Right now I'm feeling too tired to work on it, so I think I'll leave it alone until I have a rainy day or some other reason to stay home, and will start by putting everything back the way it was and work on from there.
So far it's only cost me a lot of time. If I can keep it that way I won't be too bothered, but if it starts to get expensive I'll sell it as non-working and start over with another one.