The problem is this is wasted time...
If Trump knew he did something wrong he may be impeached, let's presume for the sake of the argument he did. The only thing that would get him removed from office is high crimes against the US involving him breaking an oath of office.
If the charge happens before the office is assumed (if he colluded with Russia before accepting the office of President) there is just nothing that is going to happen. It's not a crime to collude with Russia before you're a President. After you're a President you have all sorts of diplomatic leeway and it becomes even more difficult to prosecute this. The best that could happen of this is that Mueller bates him into false testimony or perjury. Generally, a President is immune to these sort of minor convictions so again a lost cause. It would be impossible to even get this to the voting floor in a majority Republican held Congress or Senate. (some info here,
OLC: A Sitting President's Amenability to Indictment and Criminal Prosecution)
So. my prediction: Trump doesn't testify to Mueller. The End. He can avoid refuting or affirming any statements made by Comey, Cohen, and Manafort. All his testimony would do is create more evidence, so holding his tongue is the successful strategy. If Trump leaks out commentary that "refutes" any of the above he creates a potential counter-case, so no incentive to do that. He's just going to let them play out the rope until they choke themselves. There is a difference between **** posting on Twitter and testifying to a federal agent as well. His Twitter posting is completely irrelevant.