First, I think you're ignoring a category of player that will help you a lot, the very productive guy on a very solid contract. Look at Teravainen versus Aho on Carolina. Aho's cost eventually gets down close to TT's cost, but TT has a very nice contract to have no matter what. You might want to see who is on a bargain salary deal that's in FA, Trocheck is one guy to look at - costs the same as Johnsson, roughly.
So you have 4 1st round picks, and need to drop 4 players to fit them in.
Tavares & Johnsson are easy drops. Then you need to figure out who your lowest rated prospect D & prospect F are; once you do that, you put the two of them & Demko into a cage match to decide who you keep, but it's probably Demko, unless there are more attractive cheap goalies available. Demko's value is highly dependent on Markstrom leaving and not being strongly replaced with the Canucks.