Mueller's terms of reference aren't outside of any remit as they have a specific 'catch all' clause that allows them to go to "any matters that arose or may arise directly from the investigation". This means that if they discover money laundering as an example, they can prosecute that, as by any definition it's a 'high crime or misdemeanour'
https://assets.documentcloud.org/documents/3726381/Robert-Mueller-Special-Counsel-Russia.pdf
The provision for the court of federal regulations 600.4 is also fairly clear
There's actually more hypocrisy on the side of the Republicans here, notably the likes of Gingrich, Guiliani, McConnell, and perversely it would appear, Brett Kavanagh. They were all happy to support Kenneth Starr back in the day when he couldn't find anything related to Whitewater but found stuff related to Paula Jones. Ironically, the case of Jones v Clinton is now a precedent that allows for the possible deposition of Trump over the Stormy Daniels and Karen McDougall cases. With David Pecker and Michael Cohen having already testified and corroborated each other (and we don't know what Alan Wieselberg has said) it's going to get difficult for Trump when three of his own witnesses have turned evidence on him.
Also worth noting of course is the Michael Flynn plea document where Mueller has redacted two completely new crimes that he's uncovered. We haven't got a clue what these are? They've been completely blanked out. All we know is that they exist
We might however take a few guesses
About two weeks ago Trump completed the submission of his written answers to Mueller's questions. Very shortly afterwards it was announced that Manafort had broken the terms of his deal and that Mueller was voiding it, and Cohen was rushed into court to plead guilty. We have to assume that Trump has submitted false statements that combinations of Flynn, Cohen, Manafort (and what would be fatal for Trump - Wieselberg) have contradicted. The timeline fits.
There's also a suspicion that Mueller knew that Manafort was still communicating indirectly with the White House and Mueller's played him. Effectively Mueller's been selling Manafort a dummy which he's sent back to Trump, and Trump's walked right into it, thinking he was out-smarting Mueller. We'll see.
https://www.law.cornell.edu/cfr/text/28/600.4