Clemenza has a line: "...it's a lot of bad blood. Sollozzo, Philip Tattaglia, Bruno Tattaglia; Garbone,..." Who the heck is Garbone?
Seems to be a one time ghost. He's only mentioned once in the final script and not at all in the novel. Knowing that Richard Castellano loved to add his own stuff, a Throw It In can't be discarded.
After being turned down by Don Corleone for support in their narcotics enterprise, why did the Tattaglias and Sollozzo go after the Corleones? If Tom's assessment of them is to be believed, they could have been perfectly successful even without the protection the Corleones could offer, though maybe not as easily. There was no conflict of interest, as the Don pointed out. The Tattaglias could have gone on with their narcotics deal, the Corleones sticking with their own rackets, and in a few years down the line, they'd have been in a better position of strength. Going to war unnecessarily wasn't good for business.
To them, neutrality is not good enough, because it doesn't stop prosecution. It's implied that proactive help from the Corleones network of corruption is needed in order to make it worthy. At least the other families think they are not profiting enough from it without the network. Tataglia and Barzini are visibly annoyed, Vito's "selfish and unfriendly" gesture means they are taking too much risk and not gaining enough money. In addition, Tataglia is a pimp, greedy and easily manipulated (he's not invited to the wedding, he's a longtime foe), and Sollozzo is an upcoming player who is eager to climb-up and has contempt for the old Don, who "was slipping", so overthrowing the old order comes only natural to him, a generational thing, a literal Young Turk vs a Moustache Pete.
In addition, they know that Sonny thought the deal was really good (he said so at the meeting), and they correctly suspect that Hagen and Vito's other advisors think that narcotics is the coming thing. In the book Sollozzo explains that without Vito, Sonny and the Corleone family will need the extra money from drug trafficking to offset the loss of Vito's personal influence. He thinks that the Corleone family will basically have to take the deal if Vito dies, and although he does realize that Sonny will always hate him personally he figures he can avoid situations where Sonny will be able to kill him.
One more note that could have triggered the violence: Tataglia and Sollozzo may have been perfectly fine being neutral, but Don Vito sends in Luca Brasi, the Family's most feared Enforcer, as a spy (under the guise of defecting). It's possible they saw through this rather obvious attempt to spy and dig and reconsidered their neutral stance.
Why did Michael have to shoot McClusky as well as Sollozzo? After shooting Sollozzo he could have disarmed McClusky at gunpoint and made his escape. Instead of being the prime suspect in the murder of a police captain he would have been implicated only in the shooting of an ex-con drug dealer, and this wouldn't have required over a year's risky exile in Sicily.
If Michael does this, he has basically committed premeditated murder in front of a virtually unimpeachable eyewitness who (according to Sonny) has let his precinct know on a sign-out sheet that he will be in that exact restaurant at that exact time. Once Michael leaves McClusky could just finger him and he'd face the electric chair. This is why Sollozzo employs McClusky as a bodyguard to begin with. The double murder has the same effect in getting the police involved, but as the film shows, does allow the press to dig up McClusky's past.
So Michael has his goons wipe out the Dons of the other families. Whyn't they retaliate?
Remember how the Turk thought that the Corleones would eventually capitulate with Vito dead to prevent a war? Same principle, but Michael was better at the setup and execution.
Much like what Sollozzo wanted to accomplish by killing Vito, killing the heads of the other families lessens their power because the old Dons had connections that their successors aren't going to be as cozy with. In GF II, at the Senate hearings, they explain this as Michael "consolidating power". In fact, killing the other family heads was Vito's plan, as he knew the Corleone family would be weakened by his death and the only way to keep the Corleone family powerful was by a strong action like that.
How in the name Christ did Tom Hagen, a non-Sicilian, in 1945, ever rank as high in the family as consigliere? HOW?
That's what Don Corleone wanted. Who's going to tell him otherwise? In the "real" Mafia, all that stuff about blood oaths and the old country is a bunch of crap foisted off onto the junior members to make them feel like they're part of an exclusive honor society. The upper echelons see power and money as their own rewards. Look at Lucky Luciano; he handed off the "official" Mafia titles when he was locked up, then told his Italian successors that he expected them to follow his Jewish "associate"'s orders to the letter.
But weren't Mustache Petes like Salvatore Maranzano and Joe Masseria practically the racist ones, whereas Luciano wasn't?
Ever since Puzo died back in 1999, who is (or are) the literary exectuor(s) for Puzo's canon? Because are the Mark Winegardner books and Family Corleone canon?
Family Corleone could probably be considered canon, as it's based off a script written by Puzo, detailing the life of the Corleone family some ten or so years prior to the films. The first Winegardner book is allegedly recognized, but the second book resulted in Paramount suing the Puzo estate — not only because they stepped over the bounds of their agreement on authorised literature, but also because The Godfather's Revenge sold badly, and "tarnished" the reputation of the franchise. There's a Reuters article about it.