HOD said that it won’t matter, detailer said it probably wouldn’t matter but recommended writing a letter to the board if I was worried about it. If I did write a letter, would it be perceived negatively by the board? As in, screw this guy, he wanted to get out, but now he wants to stay in?
I don't see a lot of downside to writing a letter. One way or another, they're going to understand that you were getting out, then changed your mind. The voting members may not see your FITREP, but the person who briefs your record will, and will more than likely bring it up. PERS is resorting to some fairly extreme measures to hold on to talent right now - particularly someone who was previously due course. This is almost certainly going to be addressed in the community briefs. A letter from your last RS telling the board they should retain/promote you will bolster your case, IMO.
As an aside, PERS is planning on doing the DH board next month, with the potential of doing the O4 board in June so they can get back on timeline.
Edit: I guess I should ask what your last FITREP was. If it was an MP, then you might be good, as a bunch of folks made it last year w/ MPs. If it's a P, then I would definitely write the letter.