I think that he was telling the truth when he told you of the A, and I think that when he said he was unfaithful to his first W he was being honest too. He probably tried to rescind it because he felt like a dirt bag... well deserved I might add... I wouldnt be surprised to find an active A behind his wishes to D so quickly.
I told on my H,(anonymously) it got him a stop contact order and a few papers outlining the UCMJ articles that he was violating. But I had proof. I feel like it was the one thing I had in my power to do about the A, and while it may not have been the totally "correct" thing to do, I dont regret it at all. It exposed the A, and strangely enough, dirty, sneaky things dont thrive in the light of day. He to this day doesnt know it was me, maybe Ill come clean someday, maybe it wont even matter. I think he may know it was me. I wouldnt have been a very good wife if I hadnt fought for my family.
I hope that you are taking advantage of your chaplains and that you are checking out the divorce busting books. I think that its no small thing to stand for your marriage, no matter how young it is. You might be able to consult with legal aid to see what your options are once you have been served. I know in Alaska no one can make you sign anything, and it takes 2 years after the date of a legal separation to get an uncontested D.
Dont let his bad behavior force you to do anything that you wouldnt be proud of. Keep posting on here, sometimes just writing about things is therapeutic.
I guess I gave the wrong finger to the wrong man...