All is well again.
It really was remarkably like getting back online from the first system backup removal fiasco- copied over the latest known-good combination of GROUPS.DAT and GROUPS.IDX (the hiccup was in the binary IDX file, so it scrambled the information in the .DAT file regardless of how I tried to fix it, and I still haven't found my data on the exact file format it uses) and let Agent rebuild the file from the database as it sorted mail into the folders.
That's one reason I've stuck with Forte Agent even though it's the pre-2.0 version, and compared to modern programs is not at all pretty- it has a remarkably stable and self-correcting UseNet and Email database engine that shrugs off dirty shutdowns of all sorts without actually losing data. presenting it strange, but the *messages* are still there, and in the right place.