Very interested to see how this pans out. A continuing Commonwealth doesn't come up terribly often on this site, and it's very gratifying to see a timeline as well-written as this exploring the subject.
The "or is it?" has me worried. History being what it is, monarchies are often persistent buggers, especially at this point in history when republican governance is exceptionally rare. The Protectorate may not be so much of a "monarchy without a king" in this timeline, but it sounds like it's going to remain a fairly rigid oligarchy for a very long time to come.