Roberts can only be removed through impeachment in the House and conviction in the Senate, voluntary resignation. The Constitution provides no provision for the Senate to refuse to allow the Chief Justice to chair the Senate trial of the president. Furthermore, looking at the current composition of the Senate there isn’t a majority vote to remove Roberts from office much less 2/3, even if the Constitution permitted the Senate to stop Roberts from chairing the upcoming Trump trial.
If Roberts were to die or resign prior to the trial of President Trump, the President can nominate whoever he wishes to be Chief Justice but the Senate must approve the nomination before the new Chief Justice can be seated. It is doubtful the Senate will take on the appointment of a new Chief Justice, particularly one nominated by the President being tried, before the impeachment trial.
Finally, if something happened to Roberts, if you want to change the direction of the court for decades do you really want to appoint a man in his 70’s to the position of Chief Justice or do you nominate a strong conservative jurist who is much younger. Nominate a minority or white female aged 40-55 for the role who has impeccable credentials. Democrat Senators will be put in the position of opposing the appointment of the first minority or female to Chief Justice. Plus the Democrats running for President will have to go on record opposing or supporting the nomination. Their base will be pushing to oppose but if the nominee has stellar character, independent voters will not support opposing a historic appointment.
We had stellar when we got Roberts. We got two faced deep stater. You did not address Robert’s FISA court problem. If there is to be a trial the President’s defense must include exposing the FISA abuses. What happens when they call the judge to the stand?