Chief Justice Roberts will be in charge of the impeachment trial.
Mitch and his fellow senators will be little more than jurors.
Amendment VI does not apply to impeachment, which is merely a job termination civil proceeding. There is no Amendment VI requirement for a speedy impeachment trial or for an impartial jury for Trump’s impeachment trial.
The senate sets the rules.
The Senate has chosen to use the rules used for Bill Clinton’s impeachment.
That means that Senate Republicans are going to make up both the prosecution team and the defense team, and although Roberts will preside he will do so under the rules set by the Senate.
So the Senate could run the clock for days listing actual prosecutable crimes in the part of members of the house and bureaucracy which might have inspired false charges and Roberts doesn’t get to make his own objections - he can just sustain objections. If objections are never made this litany could carry up to October.
Nothing to stop that except never delivering articles of impeachment.