I am trying to figure out the credibility of the part where someone has buckled under questioning, confessed to a crime and is about to sign the written re-edited version of their confession, and is still armed.

Smells more like the FBI has just learned from past mistakes and gotten a better handle at coming up with stories to cover their own vigilantism. If I remember correctly, one of the white nationalist types they suspected of being involved in the OKC bombing also "died" during questioning.

Given the sincere belief among the FBI agents that this other Chechen guy was another guilty party in the Boston Bombings, I say there is a good solid 20% chance of him being in on it and a guilty party of something.