All of what you said is plausible. AMU is still there but I’ve never had much luck in getting historical information from them except when I contact one of the old timers directly. DCM, of course exists as another agency and I doubt if we could find anyone working there now with any sense of history.
In cases like these I usually go straight to the old time shooters. I’ll ask a few of them if they remember. I was active in shooting in 1978 and got a lot of stuff from DCM but the only '06 that I remember was standard GI issue in Garand clips. And by 1978 most everyone was shooting 7.62x51. Some of our DCM Garands even had the chamber insert converting them to the smaller cases but we always kept an original or two just so we had something to shoot the free '06 ammo in.
A million cases and bullets is a lot. You would think that some of the loaded ammo would turn up. Of course, the guys who would have it aren’t collectors and probably don’t see it as any different than any other '06.
I understand that a certain collector has a bunch of the cases. Maybe he has most of the million? ;)