1) how is it constitutional to have a trial without witnesses?
2) why isn't this travesty Chief Justice Roberts's fault?
It's not.
It's because of Article I of the Constitution... plus @GOP bad faith.
Thread.
Sec 3:
"The Senate shall have the sole Power to try all Impeachments."
Sec. 5:
"Each House may determine the Rules of its Proceedings."
There are pros and cons to these clauses. But it means the Senate decides its own rules for impeach trials
Cl 4: "The Vice President shall be President of the Senate."
Cl 6: "When the President is tried, the Chief Justice shall preside."
This tells us that the Chief Justice is not "trial judge" of impeachments. He is like VP as presiding officer & can be voted down by 51.
Entertaining to see a battle between Roberts and McConnell?
Sure, but that's not the role of a "presiding officer."