Yea, I think people opposed to a Marner trade are really downplaying how significant an asset the capspace is and how big a problem our cap structure is. Keep in mind we traded a 1st rounder, which turned into a 13th OA, to move one year of Marleau. Capspace has very significant value, plus there are frankly really good deals in free agency right now due to the flat cap.
Looking at our team, Kerfoot was our only forward after the top 4 that was making more than 2.25M last season, and that's a problem. I think you can build a better TEAM with three ~3.5-6M forwards than you can with one 10.9M foward and two ~1-1.5M forwards.
Even beyond the forwards, Hamilton is available in free agency this year, is from Toronto, is a better #1 D than we've had in decades, and is in his prime. If I'm drafting a team with no salary cap to try to contend for a Cup, I'm very likely taking Hamilton ahead of Marner, and Hamilton could be had for ~8-8.5M instead of 10.9M.
So while I wouldn't necessarily say I'd do the trade/signings you've suggested, I do think the general premise of moving Marner for a quality youngish top 6 forward + other pieces + capspace makes a ton of sense.