Zero chance Wallace's contract gets stretched.
I can see him being stretched in a scenario where the Celtics want to retain Avery Bradley while creating cap space to sign a major free agent.
I can't. In this hypothetical scenario started by the OP, Bradley isn't re-signed, and we only have $10-$12 million. If Bradley is re-signed, say at $5 million for year 1, which would at least not cause some members of this board to riot (even if I think it's low), then you're at $5-7 million cap room (probably $6 million when you account for cap holds). So then you waive Wallace, and you're at $12 million. The TPE is $10 million. What player are you going to get for $12 million who is so much better than the player you could get for $10 million that's it's worth having $4 million in dead money on the books for the next four seasons after this one? Furthermore, Bogans+Pressey+Johnson+Babb can bring you back $11.75 million in salary if all packaged together, which is pretty much where we'd be with the cap room. Yes, you'd have to do a sign-and-trade, but those generally aren't hard if you're not getting a franchise player and you're making the other team take on zero dollars of guaranteed salary. Might cost us two second round picks. That's a fine price to pay when the alternative is having that much deadweight on your cap for such a long period.
Assuming your numbers are right, hypothetically, you can trade Brandon Bass and Joel Anthony along with a first (maybe the Clippers pick) to a team with cap space for the draft rights of a foreign player who will never come to the NBA and add another $10 million-ish to cap space. 10+12 equals 22 and, hey, $22 million is around the amount of the maximum a team can offer Carmelo Anthony, so if you really want Melo, then stretching Wallace could be a way to put out a starting line up of Rondo, Bradley, Green, Melo, Sullinger.
At that point, maybe my idea of Green/Sullinger for Asik/Parsons/first might sound more attractive to some people.