Firstly I do not think this is about secularism. Turkey is still a more or less a secular country happen to be ruled by a central right - somewhat religious party.

My personal view is that Gulen was involved with the coup, but possibly jocobin nationalist/kemalist fractions in the army was also in the mix.

Zero indication at all. None. Exactly the opposite actually. Not sure about the exact numbers but a lot of the the generals that were promoted after the Ergenekon and Balyoz cases were behind the coup. A lot of secular Generals were put to jail after those cases and West didn't seem to care at all.

Are you saying Erdogan replaced pro-Ataturk generals with pro-Islam generals, and that led to more Gulen followers with control of troops than there were before?

Erdoğan and public was convinced with the coup plan in 2009 ( who knows probably they were plotting for real and gülen supporters in the army exposed it) and Gülen probably have put some (many?) of his supporters among replacements. They are extremely secretive so I doubt anybody got suspicious. I read some of the captive coup army officials statements, they are quite shocking.

Do you believe that those statements were legitimate (that is, truthful, unforced, and actually said by the officers they were attributed to)?

I'm not saying that they weren't legitimate. I have no information one way or the other. But if the regime wished to exploit the failed coup for political reasons, faked statements are one standard way to do that.

Statements are very detailed and fits the previously known inner workings of the movement. Sure there may be some modifications we never know. They just sound legit to me.

Yep exactly ! Erdogan and Gulen were close allies until 3 years ago.

A stadium full of Gulen supporters cheering for Erdogan, just 4 years ago.. https://www.youtube.com/watch?v=_bP7r0U8mfg

Erdogan's speech; "I wish Gulen was here with us today"

Actually it was not a friendship. Perhaps a mutually beneficial partnership. And things went downhill probably much before 2012, regardless of political speeches.