I can get into my main profile folder as described below:
Following my previous hypothesis, I created a new profiles.ini file with the text editor Kate that includes all the listed profiles in my .icedove folder, following the pattern suggested above. Then I saved it with a not-profiles.ini name into the .icedove folder, deleted the existing profiles.ini folder, and then renamed the not-profiles.ini file to profiles.ini. That's all.
After a precautionary reboot of Trisquel, I started icedove from the Desktop menu and got right back into my old, familiar profile, unchanged from this morning. Whew.
So far, in addition to my main profile that is so important, I can actually get into only one other profile when starting icedove, but at least I am now presented by the full list in the Profile manager popup. In two of the other four, I'm at the start-over stage enumerated before, what with having to create a whole new account, etc, which is nonsensical. In the fifth profile, it is empty, consisting of basic new, trash, etc. folders, all empty.
In the two profiles that now work, I am not prompted for any identity or authentication.
It seems that the one other profile that is now working contains most of the data that I was trying to combine into my main profile.
I think that more progress is possible with the two non-blank profiles if I just enter an existing email account & P/W.
