SPOILERS - so what was the original plan

So the plan seemed to be on track up until the father handed him the diamond. They clearly were set up for the swap.

The plan obviously got unstuck when the brother was caught and Nick handed back the diamond and the rest was improv.

What I don't get was what was the original plan? Was nick going to climb down an say look I do have it??? Wouldn't that actually be more evidence he did steal it??? They had no evidence the brother took it off Ed O'Neil so all it really proves was that it wasn't chopped up and sold. But that doesn't prove his innocence...the way it ended proved his innocence as EO was confronted in public with possession of the diamond but that clearly was not the original plan...did I miss something to do with the end game?

With the way they were talking together I assumed there were bugs... Like inside man. That Cassidy or his dad bugged the room...so he was listening if the female cop brought his story...also he was taping EO and the bad cops doing their thing on the roof and that would exonerate him but no....