trump can instruct Cohan to do anything he wants within reason since Trump isn’t a lawyer. Since Cohan was a lawyer it was his job to inform Trump of the laws.
But I think that the “what if” here is if Trump asks Cohen to do something. Cohen tells Trump that it is illegal. Trump instructs him to do it anyway and Cohen agrees. Then I believe that both are at fault.
If I were one of Cohen’s other clients, I would be deeply concerned and would want this investigation to stop immediately. I would do everything in my power to turn public opinion against Mueller. Fortunately, I am not one of his clients and this is merely speculation/fantasy.
Seems that Mueller sought and received a subpoena for all Gmail communication by Cohen dating back to 2015. Seems that Mueller has found evidence of criminality going back before the election.