I went into the Repositories view, and deleted all the previous local repos attached to this project (after confirming the most recent repo (imported from Github) contained all the project Commit history.

Still that wasn't enough.

I went into the MacOS file system, and renamed all the old project-level folders (in the folder named "git") to block Eclipse from accessing them.

Then I quit Eclipse and restarted, choosing the same Workspace I've been using for years.

THIS SEEMS TO HAVE SOLVED THE PROBLEM.

Apparently, Eclipse was still reaching into one of the older repos?? I'll never know.