Toronto optimal lineup IMO

Switch Carrick and Hainsey then move Gardiner and Hainsey down then sure.
As for LW I think it's way off other then Leivo. Especially by the end of the year.

Starts:
Hyman-Tavares-Marner
Marleau-Matthews-Nylander
Johnsson-Kadri-Kapanen
Grundstrom-Lindholm-Brown

Ends:
Hyman-Tavares-Marner
Johnsson-Matthews-Nylander
Marleau-Kadri-Kapanen
Grundstrom-Lindholm-Engvall

Everything I agree on, but Brown is not there instead of Brown.

This is meant to be optimal: not realistic. Carrick and Gardiner were #1 and 2 in Even strength defence Goals above Average and in 5v5 defence Goals above Replacement (Carrick was #1 in GAA, Gardiner was #1 in GAR). They are the Leafs best shutdown guys. Gardiner was the Leafs #1 Defenceman overall by both metrics, and Carrick was their top-ranking RD overall by both metrics. That is the best pairing Toronto can ice without stacking two of Gardiner/Rielly/Dermott on the same pair.

Marleau on the top line can add some finish to complement Nylander's playmaking and also Matthews. Matthews's shot assist numbers have been excellent his 2 years in the league, but Hyman's lack of finish has prevented those from translating into assist numbers. Marleau's playmaking is also arguably even stringer than Hyman's, so it helps that line work as a scoring line.

Johnsson is at 2LW over Hyman for pretty much the same reason: having an offensive black hole alongside Tavares and Marner is somewhat of a waste of their talents. Better to have someone who can do some scoring there.

Brown is at 3RW for his offence. Kadri's more than a checker: he was 3rd on the team in goals each of the past 2 years. Brown can complement Kadri both offensively and defensively, while Kapanen's pretty much a pure checker.

Leivo's earned himself a steady lineup spot: he's always been solid in the limited usage he's been given with this team.

You can find more elaboration on Carrick in my post in this thread: https://www.capfriendly.com/forums/thread/165248