U-126 did not save Atlantis so it's far from certain that U-44 and U-46 could save Graf Spee assuming they got there in the first place.
Still a squadron engaged in close blockade far from it's bases and close to a neutral shore is horribly vulnerable to subs, the main defence of warships against subs is steaming out of range before the sub can get a good firing solution and ships that have to stay within a few miles of a port can't do that.
IMO Prepositiong subs in case a raider needed support would have been a waste of resources, getting some there after the battle was within the KM's capability but to be any use would have rerquired the political negotiations to go very differently from how they did historically.