I do understand the way you think about the cap-situation in Boston. The only problem is that Crowder has one of the best contracts in the NBA. He is a great teamplayer and almost every NBA team could use him on that kind of a deal. O'quinn on the other hand....
I like your idea about trying to use Celtics cap-situation against them, but it's not really a huge problem for them, as they have got so many good contracts. I have even read today, that they are rumoured to be interested in Marc Gasol, which would definitely make them contenders and at the same time they have to take on luxury tax.
If we should use the cap space against Celtics we might construct a deal around Porzingis for Smart, Crowder and Bradley. That would really be great value for Knicks. Then we can move Melo to the PF, Crowder to SF and have Smart or Bradley starting as guards, and the other one coming of the bench. That would be a fun team to watch, and it might actually take os to the play-offs if we can make it work.