Not good enough. The Fallout 1 intro shows some power armor soldiers executing a Canadian resistance fighter in a ruined city, so presumably some fighting occurred, but it's usually described as being very one sided.no
The canadian gov't agreed to a mutual annexation by the US so that Fort Anchorage could be easier to defend against china. If Anchorage was taken then Canada would've been next on the list. There were resistance forces that were against this however.
Hardly, the anexation was the result of a build up in tension betwen the two nations leading the americans to bully and invade, there was nothing mutual about it. If you read a newspaper about it in Fallout 3 a general almost brags about taking Canada and calls it "America Junior" while telling people to get over it since it was theirs anyway and the anexation was just a formality.