So more on the GTA zapping system, I think the advantage of being able to zap between characters, will be to cut the fat out of making one's way through a GTA campaign. On one hand it sounds like you will be able to look at the different protagonists doing random things between missions, but on the other hand, it seems like the purpose of the zapping is to create deliberately more elaborate sequences that synchronize with one another so that the player can stay attached to the action of the story.
So the problem with the notion of applying co-op in that situation, is that the game would essentially have to make players wait in a certain spots, and perform a specific set of actions, like pieces in a rube goldberg machine, to complete the set piece mission. And if one person gets something wrong, the entire mission would have to be reset. Basically it would be like trial and error times 100 because all 3 players would impact the progression of the game.
If you don't do that the only other option is basically to start deinterlacing the missions which would defeat the whole purpose of what Rockstar is apparently trying to do. All of a sudden you wouldn't be able to force these characters to interact, and you'd have to give them their own missions---which would probably be really boring in comparison to the heists.
The whole zapping system in general isn't going to be about creating some great AI. Its going to be about creating meaningful scripted moments between the characters, while letting the player manage the mechanics. If I compare this to old Resident Evil games, they are more like a sort of pseudo precursor to asynchronous multiplayer. What this GTA system actually reminds me of, is the Active Time Event system in Final Fantasy IX---where at points in the game, when your party would disperse in a town, a little menu would pop up, giving you the option to temporarily control one of the side characters in like a mini-episode/vignette. It was incredibly effective at fleshing out side characters in that game, and it goes to what I always say about video games having as much in common with novels as they do with films---because like novels, video games are not a temporal medium and you can deliver loads of information to the player (like clicking X on an NPC to get them to repeat a line of dialogue), but like films they have all they have all the trappings of cinematic story telling to move the audience.