Is there discussion that one division might start playoffs before another division?
If so we would just have to implement a rule that you can only pick players that have not started playoffs yet.
And likely the 3rd liner from Tampa will be picked.
Using you Montreal/Calgary example - If you pick Monahan, then there would be options for the 4th line players from the Leafs or the top guys from the Canadiens. So there will be options.
Again it is not ideal with all the different possibilities.