MSL makes sense if Tampa thinks they can re-sign him. They have a young core and this isn't really their year. Callahan can help them for longer than MSL.
Granted, this relies on their ability to keep him.
MaxPac makes no sense as he is younger than Cally, bigger than Cally, and better than Cally. Montreal wants to get bigger, not smaller.
MSL is last year's leading scorer and a potential Hart trophy candidate this year. Aside from the fact it would be a public relations disaster in Tampa, and therefore veto'd by owner Jeff Vinik, if Tampa WERE going to shop St. Louis, they'd make it public and start a bidding war. And they'd get a lot more than Callahan for him (from, say, LA).
The line of thinking that supports the MSL theory doesn't take Tampa Bay's side of the equation into the consideration at all. Makes 0 sense for Tampa under any circumstances other than St. Louis saying, "I demand you trade me specifically to the New York Rangers."
Wishful thinking.