So I have re-entered all the data, with the same result - the merge won't save.
This is either extreme user obtuseness or a bug.
There is no error message or anything about the merge already in process by another user. There is a message about double-checking to make sure the LNAB is right (it is, according to the source) but no obvious way to tell the system: Yes, I double-checked.
I'm going to let it go.
Patricia, thanks for your assistance. It was a good suggestion. It should have worked.