-With Nucky and Arnold. After Mickey calls, Nucky and tells him they are sitting on a gold mine, I'm assuming that Nucky later tells Mickey to call Arnold? Anticipating that Arnold's greed will get the best of him, Arnold calls Nucky to deal and Nucky obliges to take care of the immediate problem of Gyp. Nucky also contacts Mellon so that he can take care of Arnold. But why would Mellon want to take care of Arnold? I thought his only concern was making sure he got paid from his distillery? And what's the difference between Arnold and Nucky? The only one I can see is that Arnold appears to be more stable than Nucky is.
In Mellon's first scene in the season, he's being berated for not doing much to enforce Prohibition. Nabbing someone like Rothstein would make him look good and seem like he's doing his job.