RE: Let's talk thread lock (and shimming)
If the clutchbell is shimmed properly and the bearings are good, and the clutchbell retaining screw is tight as has been stated, then the clutchbell shouldn't magically get "loose" enough to have 1mm play. Either it's not shimmed properly, or the bearings are getting loose.
I use blue loctite on my clutchbell retaining screw without issue.