Trump will make the real rules during any Senate trial. He isn't about to be silenced by whatever the RINOs in the Senate think up. Trump can, if he decides it would be helpful, hold rallies in the states of various Senators who would like to get re-elected, and invite them to come - or lose the next election.
Weak Republicans of the traditional sort worry about the rules in a Senate trial. Trump knows it is really political theater and he is a master at that.
The people who should worry are the Republicans and Democrats in the House and Senate who want to get re-elected in 2020.
but Mittens doesn’t make the rules. Willard is only one of the majority. and a newby at that.
McConnell has more control over that then Mittens.