Thinking about it, the US conquering Canada is probably the only way that Quebec would ever be able to get support for separation again. Not to mention, the Labrador area, which hated Quebecers so much in the past, that they demanded they not be considered a part of Quebec, but rather a part of Newfoundland. If the US were to conquer the rest of Canada and they had a choice between being a part of the US or Quebec, they might actually choose to side with the latter.

Not that I expect that scenario to ever occur, anyhow. Quebec is now so dependent on the rest of Canada that it would never be able to have a thriving economy, if its every borders are countries giving it the cold shoulder. It has vast agricultural lands and a very profitable pharmaceutical industry, but those heavily rely on exporting goods. Tariffs to cross borders to Ontario, the Maritimes or the US would hurt them a lot. But at least they'd have reliable electricity.