0% chance Pope Benedict had anything to do with this. Pope Benedict does not offer Pope Francis advice unless he specifically requests it.
Cdl Sarah is a very intelligent, very conservative African bishop. I think Francis probably appointed him to this because it will be something that occupies him and keeps him out of doctrine. The Africans who are being appointed as the reps at the Synod seem to be among the few raving liberal Africans that exist (especially Palmer-Buckle) and Cdl Sarah recently wrote an excellent critique of these opinions. I think he himself may be one of those at the Synod, but I’m not certain...in any case, this may be a way of trying to neutralize him.
Whatever the reason, I doubt that BXVI had anything to do with it.