I think that if Fredo was involved in killing the Tahoe shooters, there's some difficulty explaining the "you guys lied to me" phone call. If he knew someone was being murdered at the end of the night, how could Fredo have been deceived about what Ola wanted him to do? If he was told that he was helping Ola look at papers in Michael's office or something (or even kidnapping Michael), then the people carrying out the action would have to leave the compound to complete their assignment.