Don't know if New Jersey would do that deal. Personally I don't think they add any RHD in the offseason, but I bet they'd consider doing so at the deadline. They have 7 NHL level guys there right now and probably want to see how things shake out first.

Sandin for Barbashev is a pretty big loss for the Leafs IMO.

That doesn't get you Murphy even without including the cap dump of Mrazek

Add Marner going to the Devils and I’m still not sure I’d want them to take it.

Lol, now you are getting cared away. A d-man with a career -123 and a forward that looks like he plays a similar style game as Nylander isn't what a cup contender is looking for.

I'm really not.

Holl has pretty much no value when multiple equivalent 3RHD are available, so from the perspective of a team that has made the post-season once since 11-12:
Severson (looking to extend w NJD) > Late 1st
Cooley (or #2 equivalent) for 10+ years >>> 2 years of Nylander
Cooley (or #2 equivalent) for 10+ years > 3 years of Marner
Bratt at 6.5 - 7.5m for 6+ years >>> 2 Years of Nylander
Bratt at 6.5 - 7.5m for 6+ years > 3 years of Marner

There's not a single good reason for the Devils to do OP's suggested trade.
Getting Nylander + late 1st + Holl for Bratt alone would be a bad trade for the Devils.
Trading Bratt + Severson + #2 pick for Nylander + Marner + late 1st would make the Devils marginally better in the short term but would long term cap them at the level of a middling team not being able to make it past the 1st or 2nd playoff rounds.

Like I said, add Marner and I still wouldn't want the Devils to do that trade.
It's the wrong move to make.