Something that would be interesting to know is if the shooters were there at the table with Masseria (arrived with him) or if the they bursted onto the scene, while Masseria was already there.

If I have to make a guess on who the actual shooters were, I´d say that the shooters where somebody close to Luciano or of some other faction led by somebody who shared Luciano´s vision. If I have to narrow it down even further, I wouldn´t be surprised if Charlie Gagliodotto was one of the shooters, with perhaps Dave Petillo. Those two were known to have carried out quite a few murders for Luciano back in the day.