Hi,
Sorry, but I've not been able to find the smoking gun that I'm assuming you are requesting (Yes, you should question everything). I'm confident that I've seen evidence over the years for Michael's ethnicity, but just can't put my fingers on it right now. I'll keep searching.
However, what I can say is that DNA proves that Michael shares a common Shirley ancestor with Valentine Shirley (likely Michael's brother), who has documented signatures in German and whose bible is written in German (as I recall).
The German surname is actually Shally and not Shirley. You will find that both Michael and Valentine appear in early documents with the Shally surname.
Here is the Shally lineage which contains christening records.
[
www.shirleyassociation.com]
All the best
Ken