it's possible in season. You need to put Dermott and Hyman on IR as the likely wont start season with Leafs. This does not come off the cap. Need to fit 2 other players in there.

except kerfoot probably gets 3.5

Both would likely go on LTIR to start the season as they are expected to miss more than 10 games

And where's the space when they come back?

the guys who fill their spots are then sent down to make the space.

Yep...the fact the OP has Dermott and Hyman on the roster allows them (and their cap) to get moved to LTIR. They can get replaced with guys making the same or less...then when they return from injury those guys go back down.

It's tight but it could work