I have an idea about the PFC glitch....
We now know that r6v2 info is somehow attached to your gamerprofile. But I'm thinking it's probably only attached to your local gamerprofile save file (on your xbox or memory card) and NOT actually to Xbox Live...
So here's what I recomend trying (I'm at work so I can't do it myself):
1 - Sign out of Xbox Live.
2 - Delete your gamer profile.
3 - Delete all R6V1 & R6V2 save files.
4 - Clear cache (just in case).
5 - RECOVER GAMERTAG.
6 - Sign out of XboxLive.
7 - Unplug your internet modem (to disconnect from internet).
8 - Sign in gamertag offline.
9 - Boot up R6V2.
If I'm right then hopefully the recovered gamerprofile will not have any R6V2 info. And w/ no internet connection previous R6V2 info will not be recovered...
...So if all this works you should be able to start the game over, get PFC ahcievement offline, and then get credit for the achievement when you re-connect to Xbox live.