Would a couple have to specifically declare themselves or apply for common-law status, or is it implicitly bestowed?
For example, if one passes away, can the other claim common law status without a written indication from the deceased? Can the two file taxes separately, or must they file together?
Taxes in Canada, unlike the US and some other jurisdictions are always filed separately. There is no joint filing; however you are expected to declare your spouse on your tax forms.
The primary reason to declare a spouse is to claim a spousal amount if that person has no (or very limited) income.
Some help about what a spouse is from the Canada Revenue Agency can basically be summarized as living together for a year, or that you have a child (natural or adoption) together.
If you had been living together for some time, then one passed away, for that given tax year, I expect you could claim some spousal amount. But if there are deeper issues you need to clarify, then a visit to a tax lawyer would probably be worth the cost.
