A few things to consider;
1. Where would this new team play? I'm sure a new arena would be in order?
2. Would they be a division rival to the Leafs? Eastern conference rivals? New team in the west? In MLB the two NY teams are in the opposite leagues. Same with the White Sox and Cubs. in the NFL the Jets / Giants are in opposite conferences.
The thing to me, as not even a leafs fan, but just a hockey fan, is it's probably one of the most sensical places for an expansion team, especially for the health of the league. Even with diehard leafs fans not supporting another Toronto team, it would probably sell out every game. There's enough hockey fans in the area that they would just buy tickets to see a game, especially if they were affordable, and especially if they were in the same division and had a rivalry against the Leafs. What makes more sense? Expanding in Vegas or Seattle or some sunbelt city that's sure to fail 5 years later once the puppy love phase wears off, or adding another team to Toronto?
I agree with most of what you said, but the highlighted part should not be a question.
The MLB and NFL do not work in a geographical conference format like the NHL and NBA.
Those leagues have 2 conferences (AFC/NFC, AL/NL) but then have geographical divisions within those 2 conferences. Basically trying to show different playing style within close cities.
So unless the league has the team playing in Hamilton (or where ever else) but places them in the Metropolitan Division, they would have to be sivision rivals.
I just can't see a logical explanation for why two teams so close to each other would be in different divisions.