Again, why would the US annex Canada after a war. It's now going to have to garrison a huge territory of unfriendly Canadians. Sure, after a generation or two, things will calm down, but I just don't believe an American elected government is going to annex Canada. Gone are the days of manifest destiny.
In the event of an occupation of Canada:
1) Canada is returned to the Canadians. the Americans aren't stupid, it would be a costly waste of time, resources and manpower to garrison Canada.
2) Canada and Britain would have to pay reparations to the US.
3) USN would get basing rights in Halifax and Esquimault, possibly also St. John's.
4) Free trade agreement between Canada and the US. (This was the usual gripe for Americans, that the British Imperial Tariff system was hurting their business in Canada)
5) De-militarisation of Canada
It is possible that the US could claim Western Canada up to Manitoba, but I think that is doubtful. What would it gain? More farmland? Remember, this is before oil has been discovered so Alberta, Saskatchewan and BC are pretty useless (in fact, a net drain on Canada at the time).
I think it is rather fanciful to think that the US would occupy and annex all of Canada. This is the 20th Century, not 1850.