A) His cap hit is 3x8.5m, but the actual cash is 20m. 7.5m, 7m, 5.5m. Actually it's slightly less than that, as he has already been paid the 1m signing bonus the other day. So 19m, 6.5m, 7m, 5.5m.
B) Why in the hell is someone going to give him 2x10m as a UFA (in your theory) in July 2019?? There's zero chance that even if by some magical reason he could become a UFA that someone would pay him 2x10m as a 37 yr old UFA.
C) And even if he could get 20m+ over the next 3 years (again assuming this was all possible), why would he give up the guaranteed money for something that could really really backfire on him? Ask Cleary how that worked out for him when he turned down a 3 yr 10m deal with Philly to sign in Detroit. Him taking Holland on his word that he would 'make it up to him', ended up costing him millions (roughly 5m+). So while Lundqvist is in a much better spot (already earned ~85m), why would he voluntarily give up 15m+ ?? There's a reason why players sign for the most guaranteed money possible.
D) Besides... most importantly, what you're asking isn't possible, even if Lundqvist was willing to go along with it (which is extremely unlikely).
Because in fantasy land, players love assuming all the risk in deals such as this to do their teams a solid.