It's interesting that people seem to think that Marner can be signed before July 1st. He can't unless they move $4m. They either need to trade Zaitsev or move Brown and Hymen.
Until July 1st, no team is allowed to spend above the current cap for next season. The cap for next year is not approved yet, so current cap is still used.
Currently, they have $71,9m allocated to next season already, so they only have $7,59m available in cap space for next season until July 1st and the new cap comes into place.
However, that figure also includes Horton's $5,3m LTIR, so if they move that it would free up the cap to sign Marner. LTIR cannot be pre-used before a season begins. This is what forced Detroit and Chicago to trade their LTIR.
So Horton has to go, but traditionally that has cost team quality prospects (Teravainen & Crouse both 1st rounders went in this scenario), or they dropped back in the 1st round (Detroit trading back with Arizona), or an outright 1st rounder.
Now Horton has a $5,3m cap hit for one more year, but his salary is $3,6m. I believe not all of that is covered with insurance too because it is a "preexisting condition." That is why Columbus moved him, because they were paying him not to play.
I can't see too many teams wanting to pay that either. That leaves Montreal, NYR and Philadelphia as teams with cap room and able to swallow any payouts to Horton.
Arizona the traditional team for this kind of thing? Doubt it.