What does seem to have worked is deleting the actual files referenced in the 'results of last scan'. Since the items were in particularly identified outlook folders, and since googling on avast's deleting error (0x80040119) seemed to suggest that running scanpst.exe (the inbox repair tool) would be wise, I did that first. I did it on the main outlook.pst, as well as the archive.pst, and the outlook backup.pst.
I then was able to delete the outlook backup file outright, solving for the several iinfected files that were in the backup, since I can always back up the cleaned main file again. A risk, I know, but you've got your potential fire, and then you've got your fully engulfed building...
Then I opened Outlook and deleted the mail subfolders that avast referenced as containing the infected files, so as not to have to preview them individually before deleting them.
Then I went into Outlook's Deleted Items and deleted them all permanently, because for some reason, shift-delete in the previous step did not allow me that shortcut to permanently delete.
I then initiated a cleaner session with avast's cleaner.
I also started another scanning session through the simple interface, of the folders in which the deleted subfolders had resided.
I should have done them one at a time, because one of them (as it turned out, the cleaner) did find another instance of the Mitglieder trojan (which was one of the additional items that I couldn't read in a way earlier reference).
Because the scanner was running also, when the cleaner found it, I got a message from avast that only one instance of the tool could run at a time, upon clicking remove the entire infection from this computer. The cleaner was paused on the file referenced in the alert, so I first paused, then stopped, then closed, the scanner instead, thinking this would allow there to be only a single instance of the tool. I tested for availability of the remove the entire infection tool at each step, without success: same message.
I figured it couldn't hurt to do a boot time scan either, so I scheduled one of those, and then I tried the move to chest button.
The alert did allow me to move the file to the chest.
(I think)
Anyhow, I'm now going to restart & do the boot time scan, and see what happens. Perhaps in this mode, I will actually be able to Completely Remove the Infection.
Thanks for hanging in there.

I may not be part of avast's circle of adherents (yet), but I do appreciate your providing a resource that allows me to record the results of this journey so that others may possibly not tread and grieve.