First off, how is NSH going to "convince" him to not retire?
Secondly, why would NSH give MTL anything (especially that ridiculous offer you proposed), when it doesn't matter who own's his rights - if he retires early, NSH gets boned?
About the only reason why NSH would want to reacquire his contract is if MTL has already traded him to a crappy team, and he's thinking of retiring vs playing out his last year or two for a bottom feeder. Then it makes sense for NSH to at least look into acquiring him. But in that situation it's the 3rd party team (that MTL traded Weber to) that holds all the cards. If they keep him, they have a 7.8m cap hit with a 1m salary on the books. OR he retires and costs them nothing. However if they trade him back to NSH, and he plays, then while NSH has to deal with the 7.8m cap hit, they get to avoid the brutal recapture penalty. So in this case, the 3rd team could bend NSH over and get a premium return for Weber - but even then, it still wouldn't be anything like what you suggested it would be. Perhaps a 1st/top prospect as the best possible return for them.
Would getting hit with 12/24m as a penalty blow ass? Absolutely. But fortunately for them, that's a long ways off, and even better, it only actually happens if Weber retires - which means he'd be voluntary walking away from money that's owed to him. Realistically that probably doesn't happen, and in fact rarely happens in today's NHL.