With the right free agent (Dwight Howard), this team is a legitimate contender in 2012-2013 with room for Garnett and Allen. The Celtics can also use their cap space to absorb a big contract.
If not, then signing Garnett and Allen to one-year deals, creating a similar cap situation for the 2013 off-season, may make sense.
I think that the logical mid-season trade involving Ray Allen is something like him and Stiemsma to New Jersey for Mehmet Okur, DeShawn Stevenson, and a second round pick with the Celtics taking back bigger expiring contracts and effectively buying a pick. I think it possible that the Celtics could get more if they worked a sign-and-trade for Ray Allen in the off-season.
The combination of these possibilities makes me believe that not trading Ray Allen mid-season is probably the best course to take. KG's contract is difficult to trade and would mainly be used as trade ballast to bring a big contract back to Boston, but the Celtics lack additional assets to do such a trade. It makes more sense to trade Pierce during the off-season and I actually think it makes the most sense to trade him on draft day 2013, when a team can acquire him and cut him before the final year of his contract is guaranteed. We'll see how much of a trade market there is for such deals when the Mavericks try to move Lamar Odom. Dallas got a year of Tyson Chandler when they did more or less the same thing with Erick Dampier's unguaranteed final year.
I think I'd be happy if the Celtics used their cap space to sign Ryan Anderson ($11m/year sounds right) and Omer Asik (for less than Darko money) and brought back Garnett and Allen on short contracts. Probably Wilcox, since Asik seems unlikely to be able to give 30 minutes a night. And there would still be room for a few bench role players making more than the minimum. A few solid draft picks who can crack Doc's rotation and I believe the Celtics would be legitimate contenders.