It's not overpayment. It's value pretty much in line with what similar goaltenders have received in past years (Andersen, Jones, Schneider, Varlamov, even Lehner). There are multiple teams with picks in this range looking for goaltending. If the Islanders aren't willing to give up the 12th pick, that's fine (the 12th is on the upper end of the range of what would be reasonable to pay for Grubby), but don't think that the Capitals would then be obligated to take the #41 from the Islanders. The Capitals can easily turn to Philadelphia (14, 19), Ottawa (22), St Louis (29), Detroit (30, 33, 36), and Buffalo (32) to see what they would offer.
The only "fantasy" here is that the Islanders will be able to land Grubauer for less than the #12. If the Islanders don't want to give up pick #12, they aren't likely to be the winner of the Grubauer sweepstakes. Not hard to understand.