There's just two problems about that, or well one of them is a potential problem.
The first one (the potential problem) is that you'd figure WWE would have the MITB winner cash it in before the next MITB match occurs, which would be the PPV in summer, so Kane wouldn't be able to hold onto the briefcase until next Wrestlemania where I'm guessing from so much I understood of your post, that it's where you wanted Kane to cash it in right?
the 2nd problem would be that if Kane was to be able to hold onto the briefcase until next Wrestlemania, then it would be expiring on that night, because the MITB briefcase contract only allows the wrestler to cash it in before the next Wrestlemania, so it would become useless if Kane was to cash it in at the next Wrestlemania.
Besides, the last thing you mentioned, Undertaker taking his final bow, and the hugging and such, would be what I'd consider a break of kayfabe, because the only "honorable" kind of thing Undertaker does is the kneeling thing, so it really wouldn't work that well.
If you read my post fully I explained why the MITB PPV would be created, and doing that, it gives WWE the chance to sit back and see if MITB gets a good buy rate, and it not, they can scrap it for the following year, as it was only part of a storyline, and Kane would be cashing it in against the Undetakers streak not a championship so it wouldn't entirly be useless would it? which in doing so would explain the rumble winner and one of the elimination chamber winners facing for the belts.
Kayfabe? not really you could play Kane and Undertaker off as faces and it's just a brotherly rivalry and that Kane is ready to rest in peace no he's been tortured for long enough, and Undetaker could be having some problems, that he's mentally starting to fold (I posted about this in another post) and have him job, the embracing, wouldn't need to be done, but the bow (as in the kneely and thunder and lighting shit) would happen.
People say for it to happen at Survivor Series but I have a problem with that, if it was to happen at Survivor series, wouldn't people want him to retire at Mania? his stage,?