I'd rather have Carlo than Letang, especially on that deal.

I’m not sure I agree, given that Letang, McAvoy, Lindholm provides some real high end production from the back end and Letang plays a good two-way game.

Not sure Edmonton does that. Firstly they don't have any cap space, but secondly Carlo's on-ice stats don't inspire confidence that he'll cover his $4.1M cap hit. He plays 2nd pairing minutes and comp but is underwater in every Rel category, and he doesn't get buried in defensive zone starts either. He's a black hole offensively and he got buried in the playoffs - which, based on xGF% Rel, is normal for him. His two main partners this year both did much better without him than with him.

He is a very good penalty killer though and he hits a lot, but that alone isn't worth $4.1M x 5 considering his mediocre 5v5 play.

Exactly why I think he could help Edmonton defend better and Boston needs more production from the back end

But he doesn't "defend better". He's a PK specialist and Boston is better off when he's on the bench at 5v5. That carries no value.

The reason I think he could fit in EDM is he could fill a role like Larsson filled there, and more than anything this is a shuffle the deck move by Boston to move towards more offense, I feel that Carlo paired with another dfd could be a really solid shutdown pair.

Grzelcyk doesn’t fit Nashvilles needs

Larsson faced the toughest comp, first pairing minutes and mostly defensive zone starts. Carlo can't play those minutes.