Jumping into the thread, because I think this is a fun topic.
Trotsky had a chance, but was too obvious a choice - I talked of this at length in other threads, but essentially he was too much of an obvious heir to Lenin and that made the Old Bolsheviks nervous. Add to it, his personality - he knew just how brilliant he was and had a healthy ego - and his military accomplishments and there was a "Stop Trotsky" bandwagon.
Three questions: 1) if you had the choose one leader from the old guard to have the best result for the union and the world (in that order) who’d you pick? Sverdlov?
Bukharin. It could be wishful thinking and projection on the parts of Soviet intellectuals, but for many Bukharin was that mythical what-if, a decent seeming sort who was kind to folks, gave a shit about agrarian reform that went beyond "collectivize everything!" and nobody had anything bad to say about him until Stalin targeted him for destruction. Bukharin's Leninist dictatorship would not have been as bloody, in my view. People sometimes say how Bukharin's Soviet Union would not have stood a chance against Nazis, but that's pre-supposing Bukharin's Soviet Union would have still caused the Nazis as much as Stalin's did.
Part of the Nazi rise to power was due to the German Commies fighting Socialists with more energy that the right wing, due to the orders from Moscow. Would Bukharin have made the same mistake? Maybe. But it's a what-if.
2) And if you had choose one that actually had a chance?
To succeed in a snakepit of delusional monsters who were hell-bent on creating a better world of tomorrow on the bones of the old and worshiping at the altar of Robespierre and Saint-Just, you'd need a shit-heel such as Stalin, or someone with momentum and credibility. People say Zinoviev, but, erm, how do I say this politely, pigs will fly before Russia has an out and out Jew in charge of it. We're talking about a nation that invented the word "pogrom," just to clarify things.
3) if I want to know more but I’m stuck with English Dutch and German, do you have any tips for reading up on it?
Volkogonov's "Stalin" is denser than a frozen Snickers bar, but is very good. And it is in English.