Would Santo be dumb enough to be at the scene of a very public execution a couple of hours before it happened? I'd doubt it if he had foreknowledge of the hit. If you include Lansky and Santo in on the hit; then you have Genovese, Gambino the Gallos and two other people all plotting and having to have lower ranking people "in the know" to act as "buffers" between the bosses and the Gallos, carrying messages back and forth. That's a lot of people to have that knowledge and not have it leak out like Bonanno's grand schemes.

As for Jerome Johnson, there's a lot of knuckleheads who are dumb enough to take huge risks when big shots come around telling them that they'd set them up for life "for one little job". Perhaps they contacted him through one of the old Genovese heroin rings operating in Harlem? Joe Gallo wasn't the only mafia member to deal with blacks.

One reason I think they used a person not well connected to cosa nostra is that they knew that no shooter was going to get out of Columbus Circle alive and that an experienced mafia member/associate wouldn't go on that kind of suicide mission. They might have told Johnson that they'd have people in the crowd covering him so he could escape in chaos.