2) Jeff Green has accomplished nothing as a Celtic. He was bad in 2011, he didn't play in 2012, and he's been bad in 2013.
I don't think it's as dogmatically true as you make it out to be that Jeff has been "bad" this season. Sure, he won't blow you away statistically, and he's been somewhat inconsistent. But he's been a LOT better since the start of January (he's actually led the team in +/-, for what it's worth). You can't discount the "coming back from having heart surgery" factor, either. We're past the year mark on that surgery and Jeff is finally starting to look like a relatively consistent contributor. He regularly makes big plays that gets the team fired up, even if he's not scoring 15-20 points every night.
I'm more than satisfied with what we've gotten from Jeff Green this season. I'd rather have him than Gerald Wallace, Demar Derozan, or Marvin Williams . . . which basically means that he's earning his paycheck.
Also, including "didn't play in 2012" in your argument for "you shouldn't trade your starting center for that" doesn't logically make any sense. There was no way to know that he would miss a season when DA traded for him. It couldn't possibly have entered into the equation.
Not to mention perhaps the biggest problem with your argument. Jeff Green DID fill a need. Since Posey left there was a huge hole at SF behind Paul Pierce; that fact was only exacerbated by the emergence of the Heat as the major Eastern Conference nemesis. The Celtics needed a big, athletic wing to help defend LeBron and try to match the Heat's speed and athleticism. Perk would have been about as useful against the Heat as a Panzer tank in the jungles of Vietnam.
The biggest reason to criticize Danny for the Perk trade was, still is, and always will be the fact that it meant that the team was putting all its eggs in the old, often injured Shaq / JO combination at center. However, I think you can make a strong argument that the team wasn't going to win a championship without Shaq contributing, anyway.