Hard to really nail anything down until after Jul 1. Too many question marks with bubble players like Leivo (protect him, won't trade him, won't play him), Grundstrom, Aaltonen, Lindholm, and I suppose Martin.
re: Tavares - I'd rather pass on him and improve the D, and then sign some complimentary parts (Riley Nash, Letestu, Beagle, Derek Ryan). Carlson seems like exactly the Dman the Leafs need. If they can add him, and you have Borgman ready to step in, Liljegren a year later, plus with Dermott - then you can reduce Hainsey's minutes, and still insert Carrick when needed. This also reduces Gardiner's minutes, or makes him available as a pending UFA to move him.
Ideally isn't moving Gardiner for assets (picks (maybe #10?), young players, ???) now, better than the risk of him walking? Especially if you land Carlson?!?! Then maybe a smaller signing like Pateryn? Slim pickin's on D for UFA that fit the Leafs needs right now.