Ok, time for an update! Yesterday evening I received my special socket from Phuket (rimes!, thanks DHL) along with a couple of other things ordered at the same time (hemostat for cracking pressure and some second stage service tools while I was at it), so it was time to take a look at this leak. I didn't have the long hex bit socket, but was able to get by with a regular 6mm hex screwdriver bit and 3/8" to hex drive adapter in my wrench to drive the socket.
So I had to use quite a bit of force to untorque the DIN stem from the body, and it really wasn't loose at all, so I have a hard time believing that this happened by me unintentionally unscrewing the DIN adapter while pressurized. After opening, I found the internal o-ring still looked correctly centered, however, it was kind of flattened and showed a groove on its side along part of the circumference (I took a picture, but not sure it's easy to see there).
It seems to me it is more likely that this o-ring was maybe overtorqued and smashed or "bad" in the first place and cracked after a few cycles of pressurization/depressurization (more consistent with the leak first being audible upon opening the valve on a fresh tank vs mistakenly unscrewing the reg on a the previous tank while pressurized).
In any case, it clearly seemed this o-ring was the culprit. After looking at the Poseidon diagram, I noticed that this o-ring is the same part that is around the plug on the high pressure ports, so since I had taken one such plug out for my gauge, I was relieved to find out I already had a spare one begging to be used. I had a bit of difficulty coming up with a way to correctly place the o-ring , but opted to rest it centered on the filter inserted into the DIN stem and slowly screwed the stem into the body while holding it vertically and the first stage port facing downward to make sure it didn't move. Once it was finger tight, I lay the reg horizontally and finished torquing it with the socket and my bigass 1/2" torque wrench (not an ideal setup but worked ok). I then connected the reg to a full tank to test, and no leak! Seems to work just fine, IP at 128 or so with 3300 psi in there.
So, is there a good source for this o-ring (had trouble finding one before I realized it was the same as on the HP port plug)? Is this item from DGX the correct part (item 012 on that page)?
Viton O-Ring, Brown, Qty of Five | Dive Gear Express®
Also, what's the ideal way of installing that o-ring to avoid pinching it, etc?
Thanks!