The choices are, in alphabetical order, AAC (Adam Ashley-Cooper), Fainga'a and Horne. As time goes on Ant's looking increasingly uncomfortable at 13 (his defence's OK but his attack's not up to scratch); I'd start AAC (Adam Ashley-Cooper) against the Septics and Horne against the Russians. Depending where Barnes plays in these two matches (he'll probably spend quite a bit of them at 10) I'd put McCabe next to AAC (Adam Ashley-Cooper) for a good part of one match, with specific instructions to Pat to improve his body height and ball retention going into contact, and Barnes next to Horne for the other.
In fairness to Fainga'a, if Quade gets any time at 10 in these two matches Ant should play next to him at 12.
Deans should impress upon all players for these two matches their lives depend on a monumental performance from everyone. And the assistant coaches should earn their pay by concentrating on technique and teamwork.