A saga, indeed, but, finally, a happy ending.
Deleted all folders in ~Library theat were not in Test Library.
An insight for hopefully unnecessary future reference: One of those files which you didn't delete because it was in both libraries may have been corrupted.
Also, you didn't mention having deleted ~/Library/Application Support...a likely suspect.
Oh, well, another unsolved mystery with a happy ending.
I wonder if repairing permissions would have turned the trick?