I would like to step up in 'opposition' to the use of "kingdom" as well.  Here's how my thought process works:  First, do a thought experiment - picture yourself asking 10 people on the street whether Canada is a "kingdom".  I believe that most would say no, at least right off the bat.  Now, that doesn't prove anything.  However, what it does is establish on which side of this argument the burden of proof lies.  The question "how is Canada not a kingdom?", used in the argument above, is therefore out the window.  I do not need to prove to anyone that the moon 'does' orbit the earth - that's, currently, accepted knowledge.  If there were honest debate on thtat topic, it would be the other side who had to prove their point. 

 ...and the only tools we are allowed to use here are those of reference - a good logical flow based on definitions (such as "Realm = dominion = kingdom") is irrelevant - that's original research, at best.  It might be able to establish that it "should" be called a kingdom, or even that whoever did not call it a kingdom in the first place was an idiot.  What it cannot do is force Canada to be a kingdom. 

 In addition, to tag on another counterexample, even India, when under direct colonial rule, was not a kingdom.  Victoria was the Queen of the United Kingdom, but the Empress of India - because India was part of the Empire, not the Kingdom.