Confused about the theft...

Was there a point when Levene was ever approached with this plan that we saw?

Was I as the viewer supposed to assume that Aaronow was the thief, until a twist is revealed that it was actually Levene?

How did Levene become involved in Moss's plan at all, and what happened to the plan of Aaronow robbing the place?

Aaronow never explicitly agrees to participate. He appears somewhat cornered by Moss during his last scene with him at the bar, but he's still fairly defiant. It can be assumed that he finally put his foot down, called Moss's bluff and went home.

We don't see it onscreen, but Moss obviously tracked Levene down later that night and most likely made the same pitch to him. Being in desperate financial straits, Levene agrees to it.

Aaronow is only nervous the next day because he knows it was Moss but doesn't want to rat him out to the cops or inadvertently implicate himself in any way. In fact, Aaronow MAY get into some minor trouble if Moss tells the cops that Aaronow knew about the robbery beforehand. Even if he didn't participate, he lied to the cops about what he knew.