My suspicion is that Kerry's fellow officers at the time (i.e, those who became the SBVFT) demanded he leave the unit -- since they didn't trust him in a combat situation.
They may have suggested -- or Kerry might have conceived -- the notion of claiming a third PH in order to not just depart the unit in dishonor, but to get out of Dodge clean.
Kerry managed it somehow...and was gone from the unit.
I'm inclined toward this version of events because of everybody's future actions. Upon returning to the states, Kerry immediately turned on his fellows in the military -- as if he had revenge in mind. He has remained adamantly anti-military ever since.
Moreover, his fellow officers remained mum about events -- according him honor, though he returned none. But once Kerry became a candidate for the Presidency...the SBVT gathered almost immediately. Tellingly, O'Neill and about half of the group had developed strong ties to the Democrat party over the previous thirty years -- which they foreswore as being of less importance than stopping a man like Kerry.
I believe these men know more about Kerry and his circumstances than they are willing to tell...
No, when Kerry reported in-country, he went to essentially a training command, Coastal Division 14. He spent one month there before being transferred to Coastal Division 11 for three months. You are right that people didn't trust him. He spent a total of about 4 and one-half months in country.
If you haven't read Unfit for Command, you should.
"DUTY" my A$$. The man doesn't know anything about Duty, nor Honor, nor Country!