Okay, I give up. Trying again...
Since it didn't work, I decided to start completely afresh. But even recreating the bootable USB was a hassle - after trying several, ISO To USB did it. Here we go again...
Ah! Result! This time it's asked about dual booting! Now copying files...installing system...looks more promising...
Installer crashed. Of course it did. Now what?
Odd. Linux seems to be installed. Win 11 boots up normally. So does Mint.
Whaddya know? Somehow it worked!
Interesting that it puts Linux first...