I can think of three that the "rule" (like I said, I am not sure if it is enshrined as a rule or is just a much prefered practice) was relaxed in my lifetime. The main purpose of the rule is to make sure groups (and the crowds they bring) are not overlapping and that the people are spread over multiple cities...so I discount Spain in '82 who did have 3 cities with 2 stadia in use but they did that not to "reach the minimum" number of stadiums as they used 17 stadiums in 14 cities.
The other two were.
1986 in Mexico. As you noted Mexico City had 2 venues in use (Azteca and Olympic Stadium)...but only 2. I believe it was allowed because Mexico came in as a late substitute for Columbia and the number of stadia required had increased from when they hosted in 1970 (back then they used 5 stadiums total I believe).
It was, again, relaxed in 2010 in South Africa where two stadiums (Soccer City and Ellis Park) located in Jo'burg were used.....I don't recall what the reason was.
In any event.....since 1970 (when I started watching world cups) the pracitce of 1 city one stadium has been relaxed twice (again discounting the Spain experience) to reach the required number/quality of stadiums........so I guess we could use two in Toronto....if FIFA really wanted us to host we could use two in another city too (although I don't know what other city would have two WC calibre stadia - and it assumes Toronto gets an Olympics to boot).....but even if you say Montreal/Toronto account for 4 stadiums we are still a bit short and, as I mentioned earlier, countries that have hosted the tournament and started from a better position (in terms of existing facilities) have still spent more money getting up to FIFA standards than I could see we as a country agreeing to.
As for the dates.....the first CFL game this season was a pre-season tilt on June 13....right around when WCs in the northern hemisphere traditionally start. I guess you could just convince CFL teams to play at places like Foote Field and the like until the WC left town
I would absolutely love the World Cup to be hosted in this country......I just don't see it happening in my lifetime.