Well, I think that there was definitely a conspiracy to rig the election, but it is not necessarily the first time that that has happened, the Chinese and Russians have played games before. The question is, was Donald Trump and his cast in on it? There is a whole game of figuring out the electoral votes of each state in a calculus to get the right number. Trump (through Bannon?) played that game well and Hillary failed hugely at it.
Trump has said that he will appear in front of Mueller, under oath, "if" his lawyers will let him. I don't think that it will have to go to subpoena, that would imply that Mueller is compelling Trump to testify, and I think that would look bad, especially for the President. Resisting too much would be bad for the President, who after all, likes to assert that there is nothing to look for.
One thing you should remember is that Trump's family is involved because of particular kind of feudal mentality. if it was just him, he might be able to resist, but Jared Kushner is involved, and to a lesser extent Ivanka Trump too. General Flint's son is involved, which is one reason why he is cooperating so much. So I imagine that if Trump doesn't come off as his usual antagonistic self, Mueller will have a good reason not to look too closely at Kushner and Ivanka, and maybe others in his family.
But, if Trump refuses, then we may have a constitutional crisis. Meaning that we will figure it out then. But again, Trump's family (and business) is involved, and that may well be a massive vulnerability. Opponents do not necessarily have to go after just him, he has invited them to go after his family but involving them in his administration.
But again, I think it is important to remember that there are two investigations, although I would say that the independent investigation FBI is more important than the Congressional one (although Trump could conceivably fire Mueller, he cannot do anything to stop the Congressional one).
US Marshalls are in the Justice Department, but not in the FBI.