Gary:

Your help is still very much appreciated.

FYI, a kludge, but one that works (on minimal testing), is to delete the worksheets indirectly calling:
Application.OnTime Now + Timevalue("00:00:00"), "DeleteWorksheetsMain"

The time value of "00:00:00" still worked and produced no noticeable delay. I apparently assumed correctly that this would allow dismissal of my custom popup before the offending action occurred.

Of note, I found that the problem doesn't just apply to worksheet deletions, but also when you just change worksheets. I think it's actually the forced worksheet change that causes it when the sheets are deleted. I haven't the time because it's very late to triple check that observation, but that's my strong impression.

All the best,

Greg