I'm sorry to say that I can't help with any of the outstanding mysteries. I do, however, have one of my own to toss into the ring.
Back in the mid 1960s, my brother had an SF novel that contained two separate memorable themes. One was that Stonehenge was not a human-made ruin, but a long-disused landing site for alien spacecraft. (That may be a pretty common idea, though, so I wouldn't expect that to uniquely identify the book. BTW, this was years before Chariots of the Gods!)
The second one may narrow it down a lot, though. Our hero ends up on an alien world populated by humanoids... with a difference. They're immortal, but they periodically undergo a "reset" -- they lose all memory of their past life and start over. They had learned to store their memories and reload them after the "reset". However, any sort of criminal offense had a simple punishment: the culprit was walled up in a cell until after the next "reset", and didn't get reloaded with their old memories. They were sort of exterminated without being executed, if you see what I mean.
Naturally, our hero commits some trivial offense on this world and gets himself walled up (the locals didn't realize this would have fatal consequences for him). There's a memorable scene in which he manages to extract himself from this rather gruesome fate.
I have no clue as to the title or author, and neither does my brother. Any bells ringing out there?