You know, I'm coming into this late, but it seems like you're saying "If they could do it, could they do it?" which is circular.
Could they hold it? Well, you are making a great deal of assumptions to start with which, while you can brush away by saying they are not part of the question, are indeed material. You can't possible hold something if you can't first take it.
Posing questions like this where you ignore all sorts (not just a few) causally related events is like saying:
What if Germany had the A Bomb in 1940?
What if Japan had not attacked the US?
What if Poland had allied with Nazi Germany?
I mean, once you upturn the can of beans, you can't choose which ones spill out.
Also, it is important to consider causal relationship when you suggest Hitler mount the conquest of England and not France. Why would he attack one and not the other? For that matter, why would he not engage in a war with Soviet Russia? All of these answers will determine 1) How Germany was able to carry out a successful conquest of the areas you describe and 2) If they would be able to hold it indefinitely. Also, why are you restricting it to only Canada and the U.S. intervening?
I would back up and look at the whole scenario from an earlier point of view. Then, use prewar historical data to help determine reactions to Germanies actions. Also, you will need to look at political and diplomatic relations between all of the involved nations. From there, propose minor changes in history and project their long term effect. You will better be able to get an answer to your question if you construction a reasonable and feesible scenario first.
Just my meanderings...pay no attention to the man behind the curtain.