(disclaimer - not a flight surgeon - civilian surgeon & pilot, corpsman from a long time ago - but I know a lot about pneumothorax)
Spontaneous pneumothorax (collapse of the lung) usually happens because one has "blebs" in the lung that can rupture & release air into the chest cavity, collapsing the lung. The first episode will probably be treated by putting a tube in the chest cavity for a few days. This type of pneumothorax is prone to recur if one has more blebs, and in the civilian world one is grounded until surgical correction if you have a second episode because a pneumothorax can cause sudden incapacitation, particularly in flight. I am guessing the military standard is even more stringent. But the good news is it sounds like you don't have this (but it sounds like this might be what NPQ'd NavyOffRec's other candidate).
Traumatic pneumothorax is very different. It's also usually treated with a tube in the chest cavity for a few days, but it is unlikely to recur if there is normal recovery from the injury and no pre-existing disease of the lung - which I am sure is why it's shown as not CD after a 1 year wait. With yours being four years ago and a normal workup, I don't understand why you would have been DQ'd. If there is something you don't know about that is a problem, it's most likely on the chest xray. But IMO you shouldn't give up on this. It seems to me that (assuming there isn't missing information you don't know about) you may have been DQ'd in error. If the DQ stands, I would think a waiver would be very possible (again assuming there isn't missing information you don't know about). Spontaneous pneumothorax (especially recurrent) is a serious aeromedical hazard, a well healed traumatic pneumothorax in an otherwise healthy person usually isn't. good luck.