...”He wants to give the president an opportunity to explain any ambiguity and any impression that he was favoring a foreign adversary...
BS.
And pure BS. Mueller doesn’t even have a specific crime he is investigating. Trump doesn’t have to get interviewed, or even plead the 5th. He can literally ignore Mueller.
Imagine a cop walking up to you and wanting to interrogate you.
You ask him, “what are you investigating”. He says “nothing in particular, but the old lady down the street suspects you are up to no good and I want to gain an understanding of your life down to the smallest details”.
You have no obligation to continue the conversation.