You know what, I like this offer. It's crazy enough to work. Now, there is no way Montreal signs him to the 2 year deal and lets him walk in UFA. Giving up 2 1sts should at least buy you some long term stability. Is Mantha worth that amount? No, but he could be. The potential with Mantha is legit, he can create offense with anyone and is also a solid defensive player too. If he stays healthy he is an 80+ point player on Detroit, the worst team in the league. Detroit fans wouldn't sign him to that deal, it's very rich for his services, which is exactly why it might work. The Aho deal needed to be two million dollars more a year in reality.
IMO, Mantha is the perfect player to offer sheet, he's talented enough where it could be worth it, yet unproven enough that his current team might not match. If you can identify a player that will make your team a Stanley Cup favorite if you get him, giving up 2 or more 1st round picks isn't a big deal, they would be de facto 2nd rounders anyway.
That said, I'm reasonably confident we give Mantha his big contract in the next month, but hey, this is the kind of logic fans need to even reasonably speculate on offer sheets.