That Chihiro was a fruitloop who would go as far as it took to hold on to the power he'd inherited, of course, is less open to question. It wouldn't surprise me if he was the person who actually oversaw the building of the original OBS, and it's plausible that he actually pushed the button. It's possible that he did it without waiting for further orders, but not probable it was entirely without orders. Once in control, it's very likely that he introduced changes to the Plan to make it more congenial to himself either because he was simply a natural reign-of-terror sort or because they assured his hold on power. Or both, of course.
That deviation from the Plan may have been the cause for the mutiny, if some saw it as undermining the long-term viability of the COGA. Alternatively, of course, it may have been good old-fashioned self-interest or self-preservation in those who saw Chihiro's rule as a threat or an opportunity. "Langhorne wouldn't do that! It was all Chihiro!" strikes me as wishful thinking and/or a justification and means of attracting supporters, depending on the motivation of those saying it.
I am not certain at all about this, but I recall some doubts that Langhorne ordered the destruction of Alexandria. Now let's assume for a moment that Langhorne didn't order the strike, but Chihiro did.
Commodore Pei gains admittance to see Langhorne and confronts him about the strike. Langehorn states he didn't order it. Commodore Pei doesn't believe him and nukes them all. There is no record of the exchange, nor anyone alive to recount what transpired. Chirhiro and Shueler could have supported any interpretation of the events that transpired. Further assuming Shueler didn't know the truth, Chihiro could have planted enough doubt to pull Shueler along.
Toss in the fact that forces loyal to the Commodore had secreted away sufficient production capacity to almost pull off the revolt. Had they been able to deny Chihiro and Shueler the use of Hamlicar, they would have won. Now, Shueler can believe there was indeed a conspiracy against the command team. He can believe this because it was true, there was not just one but two conspiracies.
By the time Cody comes to Shueler with his concerns, the rebels were pretty much defeated. Shueler either believes he has no options but to accept Chihiro did indeed start this war or is given no option. In either case Chihiro kills Cody and Shueler finds a way to give his descendants the Stone and the Key. Shuler sets his descendants on the mission to keep the CoGA honest and lets them know there will be a Return.
Still not sure what will Return. If it is Chihiro's digital personality, why not leave it there to be ruler after his body dies? If a PICA, again why not leave it? Biological bodies couldn't survive long enough. If it's an AI, why not a digital copy of one of the command crew?
The one scenario I see, is that Shueler got a concession from Chihiro to keep either one or both of their personalities in storage until the Return. Those personalities could have been stored in the Temple or in Hamlicar. If Shueler succeeded in retaining Hamlicar, there is a good chance that he was largely convinced Chihiro's plan was not best for Safehold. If the personalities are stored in the Temple, Sheueler was likely still open to accepting Chihiro's plan.
In any case, I can't wait to start the next story arc.