I'm not generally big on conspiracy theories. I consider myself to be pretty level headed.
I also think there's some validity to the idea that Mangini played a role in the Jets drafts when he was here, and did a decent job. A guy like that, I'd consider giving a job in te front office. Sure, maybe you don't hand him the GM job right off the bat, but some sort of assistant job? I think that's very reasonable. Especially if you think he had a hand in pushing for Revis and Mangold.
The fact that he hasn't gotten any bites suggests to me that he's been black-balled. It happens. Either it's because he's a rat, or he ratted out Belichick and no one wants to cross him.
Spygate was a huge, unprecedented deal. I think it blew up in Mangini's face.
I get what you're saying, but I don't think it's as cut and dry as the "rat" angle. Fact remains the Jets HC position was not the last coaching gig he had. Someone else - albeit, Cleveland - hired him while Spygate was fresher in GMs' minds.
On the other hand, it would stand to reason that someone might hire him as a position coach or DC except no head coach
wants a potential rat working under him. I think if he showed more success as a HC he wouldn't be blackballed.
Given time I think he'll get another NFL job. He can always play the "I was younger then and it was stupid" angle, and it probably isn't far from the truth anyway. Some will never take that leap of faith with him, though.
I think more than Spygate, there are 2 more compelling reasons he hasn't been hired again:
1) poor record of success, even just as a DC with the Pats
2) he was a dick who too many players despised as a coach. That's a tough hump to get over in light of #1.
But you could be right and I could be wrong. There's a first time for everything.