A few points:
(1) I have reviewed the chronology and, at first, I thought Kerry was required to actively drill monthly in the USNR from Jan 1970 to Feb 1972. In fact, however, I don't think this was a requirement. If he had blown off the drills then he and his fellow-travelers would have no ammunition with regard to Bush's ANG service. Regrettably, however, I don't think he was required to drill in those two years after leaving active service.
(2) I believe he was issued an Honorable Discharge in 1978. This is still unusual, as it should have been issued in 1972. I, like you, suspect he got something less than an Honorable in 1972 (due to his disgraceful post-Vietnam activities), and finally, during the Carter presidency, he got it upgraded to Honorable.
I would bet that his refusal to sign the Form 180 is based on this.
The discharge is not received until the 6 year commitment is up. I was released after 4 years active under Honorable Conditions on the DD214 but the HC came 2 years later. Which in Kerry's case should have been 1974, I believe. If it was 1978, something was amiss and probably got cleared up under Jimmah Carter the "pardon the draft dodgers president".
I'm with you, I think some clean up began shortly after Carter took office. Which one of his papers show something about coming through some kind of attorney's office? I thought it was his '78 discharge.