The reason teams get these young RFA players at what seem like such high initial cap hits relative to the player is because they go 8 years. They overpay in the short term in hopes of getting a good deal in the mid/long term. If the Leafs go 5-6 years with Marner and Nylander, they can shave significant money off of their AAVs.
Pastrnak for example signed for 6.66M coming off of a 34 goal 70 point season. That was 8.89 percent of the salary cap at the time. So that's roughly a 7.1M cap hit right now. There's probably other examples as well. I could see Toronto getting Nylander at 6.5M-7M at 5-6 years.
Next year, Toronto could then sign Marner to a 6 year deal in the 7.5M+ range. Six years would be important, because it would mean that when the deal is up, the contract of Tavares would be expiring. That would increase Toronto's ability to keep Marner as a UFA while allowing them to benefit from a lower AAV by not buying up two extra UFA years on his initial extension.
I'm assuming it is a given that Matthews will probably get 10M-11M x 8 years.
It's an interesting balancing act cap wise, but I think people over rate the trouble they might be in. I don't think the issue is an inability to keep all four big forwards, they clearly can keep them, the issue is would they be better off flipping one of those four as a trade asset (and to open up cap) to bring in a stabilizing presence on their back end. Do they want to be top heavy up front, or more balanced?