Not following the logic here. Mueller needs to prove his case while the investigation is still ongoing? That is not how any other criminal investigation works. Not sure why an exception should be made for POTUS.
Also, a "process crime" is still a crime.
criminal investigation means first there has to be a crime to be investigated.
No such crime was identified when Mueller's investigation was initiated.
Primary objectives of a criminal investigstion include both the determination of (1) if a crime was committed and, if so, (2) identification of the perpetrator(s). Regardless, there is no dispute as to whether crimes were committed — DNC and Podesta communications were both stolen, along with two guilty pleas and two indictments.
Even Constitutional law professor Jonathan Turley who leans left has an interesting things view on if a crime has been committed:
http://thehill.com/opinion/white-house/366824-for-president-trump-2017-ends-largely-where-it-began-free-of-charges
Just this week, constitutional law professor Alan Dershowitz (who says he proudly voted for Hillary) said now the left "couldn't criminalize political differences" in regard to President Trump. . .
https://www.realclearpolitics.com/video/2018/01/08/dershowitz_the_left_couldnt_criminalize_trump_now_trying_to_psychiatrize_political_differences.html
Agree, there are plenty of plausible legal arguments as to why Trump cannot be prosecuted. And I still haven’t seen any direct evidence of Trump himself being part of a criminal conspiracy (obstruction/Comey firing may be another matter, however). None of this precludes him from having to be interviewed as part of the broader investigation and current prosecutions. Hell, if President Clinton had to give a deposition in a civil matter over sexual misconduct, then I don’t see any realistic precedent for Trump avoiding this one.