Wikipedia is not the only source which gives the definition of the term commoner. Others include
Peerage - Everything on Peerage (information, latest news, articles,...) and even the parliament's page
http://www.parliament.uk/commons/lib/research/briefings/snpc-02666.pdf. The latter says:
"Under this ancient procedure, all persons, whether peers or commoners, may be prosecuted and tried by the two Houses for any crimes whatever"; "If the accused is a peer, he is attached by order of that House. If a commoner, he is arrested by the Commons and delivered to Black Rod." William is neither peer nor sovereign, therefore he is a commoner, just like The Princess Royal and York girls. I hope that the parliament site's "credentials on the use of Cmmoner in England" are good enough to satisfy your criteria.
Once again, I used the term "peasant" precisely because it is comic and because I couldn't use the term "commoner". I am really surprised that someone actually thinks that I considered The Duchess of Cornwall a peasant
For the same reason everyone else is posting here and in threads such as "Possible names for Marie and Joachim's baby": fantasies and wishes. We all know that William won't marry someone just because we want it, but some fantasies can't hurt anyone. Besides, this place is for discussions and I don't see anything bad in discussing who would be a good wife and consort for William.