A lot of things:

-That dosent get you Manson.

-JVR dosent have as much value as a rental

-The Ducks likely could not resign JVR unless Vatanen was the one traded for JVR. Even then it would have to be a reasonable extension + more.

-Ducks aren't trading Manson unless there is a HUGE overpay. Such overpay would be a player like Marner or Nylander. Toronto obviously doesn't want to move either and the Ducks don't want to move Manson, thus why a huge overpay would be needed for Manson to be traded.

-Manson would not be playing on the bottom pair in Toronto.

I think you got a few points off there.

Yes, Manson would find himself on the bottom pair unless he made good chemistry with Rielly, and I doubt the Ducks could afford JVR long term without moving cap out in return.
But I don't truly see your points about JVR not being as valuable since he's a rental. Let's not forget rental trades of the past (Hanzal, White and a 4th got Downing, a first, second and a fourth - Ladd got Dano, a first and a conditional pick - Staal was acquired for two seconds and Saarela and even Vermette got a first and a prospect defenseman) and all of those trades were for older players (30+ except for Hanzal) and were producing well but not 60 point pace (actually all of the players listed above were tracking for just around 40 points) and most importantly, they all were acquired for a possibly long playoff run, but at the least just around 20 games. If this trade happens in the pre-season I could see JVR (fresh off a 60 point season playing around the league average for a third liner) getting quite the return, especially if any salary is retained.
Also, I've seen some of Manson's play, but with Montour coming up this season and Vatanen still in the lineup, where will Manson slot in long term, if one of them isn't moved? I could possibly see the argument of Nylander being involved in a LARGE package trade where the main players being moved were Montour and Nylander (granted Montour is having a solid season, but I can't for the life of me picture a trade where Nylander is equating to the pay of Manson. It just doesn't make sense to me. I see that Manson is a 25 year old, big defensive minded RHD, but is that really equal to a 21 year old speedy right winger coming off a sixty point rookie season?
Also, if Manson is really as good as you say - to involve possibly Nylander - wouldn't that mean he's going to command quite the raise at the end of the season, and put the Ducks in a cap situation? And Nylander would definitely command a heck of a lot more than either JVR or Manson (not to mention he's a righty and the Ducks have a LOT of rightys)

I also, regarding the trade listed above, can't see the value of JVR and Carrick being moved for Manson and a third. Carrick is a younger RHD who is coming off a fairly stable defensive rookie season, and could have a lot of upside, and JVR (as I said above) is a 60 point winger who can really contribute espeically if he's playing with Getzlaf and Perry

I hate these kinds of posts so much. Potential future core players WILL NOT be in the press box, any smart organization will get their future core pieces playing time elsewhere. PLEASE STOP & also, OVER the roster limit. & also, NOT rolling 5 forward lines..