There are unanswered questions under the single shooter theory. Are Afghan officials being paid off? How did a single man exit a camp when the army has exit guards and procedures? How far apart are the villages? Did the shooter bring his own gasoline? Some further info is here.
On the other hand, if there were multiple shooters, there are also serious questions against such a theory. How could one man be induced to take the blame for all? Wouldn't there especially be records of any larger scale operation that left the camp? Won't the defense attorney seek such records? Couldn't he bring army witnesses into court and tear apart any concocted story? And granted that the army has an incentive to go with a single shooter story so as not to see the whole Afghanistan operation besmirched, who would have and could have arranged a single shooter coverup story so quickly? And aren't the risks awfully high of doing a coverup that might fall through?
Due to these issues, I have stronger reservations concerning the multiple shooter theory than the single shooter theory.