Very interesting scenarios, Wayne! Yes, Barzini is the key player. In my view:
Sol had needs--to operate with the sponsorship of one of the Five Families, $1 million in cash, and police/political protection. Any of the Families could sponsor him, but only Corleone had the cash and the police/political protection.

But Sol also had something to give to his partner: the money his partner would earn would make the partner stronger because he'd be able to buy more police/political protection. Corleone wouldn't be interested: he was already on top with rackets that weren't as risky as Sol's. So, I believe, Sol contacted Barzini first because Barzini was younger than the other Dons, impatient to be Numero Uno, had more to gain from a deal with Sol--and would give him better terms. Barzini replied, "Your logic is good. But you still need Corleone's judges and politicians. If Corleone knows that I'm your partner of record, he'll say no. Go to Tattaglia and tell him anything he wants to hear. He's a pimp. Corleone isn't afraid of him. But I'll be your partner--your silent partner--and I'll have your back at all times."

Barzini couldn't lose. If Vito said yes to Sol's deal, the money Sol channeled to Barzini made him richer and stronger. If Vito said no, then Sol would try to whack him. If Sol succeeded, Barzini automatically would become Numero Uno. If Sol failed, the Corleones would come after Tattaglia.

In any case and in any scenario, the money to be made from drugs would create uncontrollable greed. Whomever was perceived as making the most money from drugs--Sol, Vito, Barzini, Tattaglia--would become a target for the others. And even if Vito agreed to protect the drugs trade with the proviso that the others had to use discretion, keep it respectable, use couriers with clean records, greed would cause the others to violate those terms. The moment Vito refused protection, he'd become the target.

Moral: crime doesn't pay.