In the event Mueller does find substantial evidence of Trump's criminality but feels obliged to

[TomSlick]

1. No.

Prosecutorial discretion is - well - a matter of discretion. A court will not order a prosecutor to bring a charge.

If Mueller believes that he lacks the legal authority to seek an indictment of the President, he ethically ought not ask the grand jury to indict. It's possible, albeit unlikely, that a grand jury could return an indictment over the prosecutor's objection. Grand juries will generally follow the legal advise of the prosecutor.

I personally see nothing in the Constitution to say a sitting President cannot be indicted. However, lawyers who are better constitutional experts than I think it an open question. My perception is that Mueller is enough of a "company man" that he will follow existing DoJ Office of Legal Counsel opinions that an indictment would not be appropriate and that the only available remedy is impeachment.

I think that if Mueller and the grand jury believe there has been a crime, there ought to be an indictment to get a SCOTUS ruling on the question whether a sitting President can be indicted - that's the only way to have a definitive answer to the question. However, my bet is that Mueller will follow the DoJ opinion.