Cohen telling Costello he was at wits end and didn't have anything on Trump would have been perfectly natural for him to do at the time. For one he would have been hoping Trump would help him out. And for two he would have expected Costello to share whatever Cohen said with Trump.
And that business of Cohen stealing money from Trump Org. because he didn't feel he was being paid enough should not matter to the guts of the case either.
Correct me if i'm wrong, as far as i know the defense has not been able to show that anything Cohen has said about Trump in the trial has been untrue.