I checked the shell latch very carefully and wouldn't change it, as it appears to work very well, but with minimal shell contact.
The shotgun is in very good condition. Parts don't appear to be worn and the fact that it functions every time with rounds in the magazine, but only malfunctions on the last shot is nothing I can cure without a bunch of new parts to try, so I'll give up on this one and let the guy send it to the factory or to an authorized repair station.
Thanks for all your good advice. I learned quite a bit about this shotgun model.
“God save us from people who mean well.”
― Vikram Seth