AT, It is not "within his executive powers" for Trump to order Comey to back off of Flynn.  Had he done so, I would call for his impeachment in a heart beat.
He didn't do that, but he came darn close.  Comey didn't think he had crossed the "order" line by asking what he did.  That is why he didn't mention it to Congress.  But now that he has been fired, he is having second thoughts.
Now the NYTimes - never my favorite source of info - is reporting that Trump also asked the DNI and the other intelligence leaders to say there was no investigation.  The ice is beginning to break.