If, in fact, Trump is directly compromised by Russian intelligence and it becomes clear this is the case, there are constitutional and legal mechanisms in place to deal with this and succession.
Number 1 on that list is impeachment. If it is revealed that Trump colluded with Russian government officials during the election, then political will to impeach AND convict in the Senate will be massive.
In the event of Trump’s removal by the impeachment process, Pence will become the president.
Should political will not be great enough, it takes eight members of the Congress and the Vice President to remove a sitting president and there should be a significant amount of political will to carry through with this option should the impeachment path fail.
Should it be revealed that Pence was a part of the collusion with the Russian government to affect the outcome of our election, political will to remove both will be so vast that any member of Congress voting against impeachment and any Senator voting against conviction will not be a member of that body after their next election, and the Speaker of the House (Paul Ryan) will become the president under US Code, Title 3, Chapter 1, § 19.
IF through some amazing bit of criminal conspiracy Paul Ryan is also found to be part of the collusion with the Russian government, then under US Code, Title 3, Chapter 1, § 19 the President Pro Tempore (Orrin Hatch) becomes the president.
I simply do not see how a criminal conspiracy like this could go any deeper than Ryan, so that is the ultimate end point under a potentially massive criminal conspiracy to use the aid of a foreign government to steal an election.
Regardless of all of this, there is no constitutional mechanism to ever run the 2016 election again in a “do-over, or to declare Hillary Clinton the actual winner of 2016. Looking for a “do-over” or to have the courts install Hillary Clinton as president is as ridiculous and asinine as the wet dreams of the Birthers to remove President Obama over a birth certificate.
We have a constitution and laws and they are perfectly capable of handling even the worst case scenario in all of this.