How should one handle miscommunication being abused?
Technically Andrew cannot act at step 3 because at step 2 targets and modes were not fully defined so Subtle Strike wasn't technically on the stack. 

So, since Andrew acted without priority, the steps should be rewound to Subtle Strike being cast.  It would then need to be fully cast before priority is regained by either player. 

Now, assuming that there were no modes and only a single target, I would actually say that Nick implied that he chose the thopter as his target, and since Andrew accepted the shortcut (by acting like it was on the stack and casting his spell) that Nick's choice is set. There's an old article from Wizards about "false information", but there might be something more recent.