I'm not saying that such a plan would be easy to execute, but it does have the advantage of being completely unexpected. I mean, if you suddenly recieved word that the was a large Nazi invasion force was driving southward through Manitoba, your first response would be "what the hell are they doing in Manitoba?"
Don't forget that the Allies were caught completely off guard during the Battle of the Bulge precisely because they assumed that it was beyond the Nazis' logistical capacity to launch a winter offensive and because the senior Allied commanders overesimated the impassibility of the terrain.
The same was true of Pearl Harbour. The Japanese appeared precisely where they weren't expected to be.
The difference between those plans and your Hudson's Bay invasion is that the Hudson's Bay operation isn't just unlikely, it's a logistical nightmare, especially when one considers the lack of Port facilities, Ice, and distances involved especially when it comes to Logistics.
No, if the Nazi's really wanted to Attack Canada there's only one way they really could do it and that would be going through the St. Lawrence Corridor.
This operation would of course have to be preceded by an operation in the Maritimes, a sneak attack on Halifax/St. Johns/Gander and the occupation of those zones plus Cape Breton to keep supply lines open. Nova Scotia would be bypassed in favour of securing PEI and New Brunswick. Then with the Maritimes pacified the Wehrmacht moves down the St. Lawrence landing in the Gaspe and siezing Quebec in a daring airborne/special ops maneuver. With the St. Lawrence Sea Way open the Panzers start rolling towards Ottawa and from there straight to Vancouver.
However all things considered it isn't
That much more plausible than the Hudson's bay idea. A little, but not much.