Where in the Constitution does it say that all Senators must be present for the entire trial? If one Senator says “sorry, can’t make it”, does that mean it doesn’t happen?
It doesn’t. It says “members present” but the rules the Senate has established require the presence of all Senators. No vacations, no absences except for health reasons. They could, of course, change those rules by majority vote, which really requires 60 aye votes, again according to cloture rules. I doubt the majority would permit it. I’ve heard that Democrats are planning to try to cut down the number of Senators present to reduce the 2/3rds number by having a large portion of their members play hooky, so they don’t have to have that many Republicans jump ship.