Canada's immigration laws do not presently account for the possibility of a same-sex or common-law spouse. The Immigration Department, however, does recognize that such unions exist, and have implemented policies which may allow for immigration on the basis of such.Initially such applicants are considered in independent categories. If the applicant does not meet the requirements of such, then Humanitarian and Compassionate grounds are considered. In the latter case, documentation or other corroboration of the relationship is critical, as you mention.
It is not necessarily the case that such applications must be processed from within Canada, although your presence therein may facilitate the evidence of the relationship.
There does exist a self employed program for applicants with prior experience as such.
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Campbell,Cohen - attorneys at law
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