The key to any of these creative 1 year plus X deals is that they aren't binding. They aren't actual legal agreements. You can't have a contract stuffed in a drawer that is legally binding on the club.
If he plays one year at the max, and he gets injured, then that long-term agreement in principle with Toronto goes away. The league makes sure that in such a scenario, the player has to assume all the risk. It's really one year guaranteed at the max with Toronto, versus $80M or $88M or even $96M over eight years guaranteed with the Isles. Or a seven-year deal with anyone else.
There are creative proposals that the Leafs can come up with, but one gigantic year followed by a long-term deal isn't one of them. It's not real.