I don't see large scale renovation viable. If Buffalo was temporarily annexed by Canada, maybe. But playing a whole season in a minor league stadium in another country? League wouldn't go for it. Only way I could see it is if it was a very short timeline, starting in September (so the Jays have to play entirely on the road in the season before) and ending in May (so the Jays have to play April entirely on the road). It would be impossible to do anything more than minor from October/November to March. You need at least half a year. Probably more.
I still think the redevelopment of Downsview is out of scope for building a new stadium. It's one of the least affluent areas in the city of Toronto, and ignoring the cost of redevelopment, you also have the social considerations of gentrifying a lower-income part of the city. A city that is already increasingly seen as unliveable for anyone outside the upper-middle class.
The Portlands is at least a pure industrial/commercial area where redevelopment isn't going to put upward pressure on cost of living for low-income persons. The residential areas near the Portlands are already extremely expensive. Not to mention the Portlands gives the opportunity for a lakefront stadium a la San Francisco. Still, due to the cost of redevelopment and transit connections, the CNE grounds (which I suppose would include Ontario Place) still sound like the best option to be. Transit connection, little need for redevelopment, space for a stadium, good aesthetics, good optics, and more.