I don't know Canadian visa law, but I would assume that your US firm needs to have an office in Canada to be able to sponsor a Canadian work visa for you.
Sorry if this is abrupt, but it sounds like you are frequently posting your interpretation of legislation. US immigration realities are frequently different. Even if you've had your own experience, it would probably be more useful to OP to hear from the expert rather than one person's experience.
IANAL, but I've done a large amount of research into this, and I think an immigration attorney would find the answers I've given so far to be quite accurate.
Not sure what you mean by "US immigration realities are frequently different" -- all immigrants are subject to a same set of US immigration laws.