the ducks aren't telling their player, you're not wanted, make an extension deal with leafs, and of course if that doesn't happen you're welcomed back to our ducks family. stop with these non happening sign and trades.
leafs probably don't want sliverberg long term anyway. montour and kase has better possibilities.

Leafs wouldn't want him beyond this year, maybe for a play off push, but I would flip him one more time for some goaltending depth for the playoffs or fourth line center depth.

Once again, zero reason to make this trade. Leafs need to add to their roster not subtract anyone.

Why don't just trade for Pac ? Like you said, MTL will be out of the run sooner than later. So keep your RHD in Montour(so you don't need Petry) and add Pacioretty instead of Nylander who, lets be honest, won't be trade. Pacioretty will cost you Larsson, a 1st and maybe a little more but you'll have a better depth by keeping Silverberg.

Given the age and stage of the Ducks, they've got a 1- to 3-year window to snag a Cup or two before Getzlaf and Perry buy the farm. Adding Petry AND Pacioretty would make tick a lot of boxes. If the deal were rich enough for Montreal, the Habs could even sign Pacioretty and then eat 50% of the contract, which would allow the Ducks to keep Silfverberg.

As for waiting 20 games, no GM would do that. If this deal is worth doing, it's worth doing at Labor Day so that the teams can use training camp to gel.

I don't expect Petry to move... Pacioretty to ANA at the TDL is something possible IMO. As rental, and after that, it'll be ANA GM's job to find a way to sign him.