Sure he can. “Case dismissed, here’s why”. Done deal.4
Not his call. As the court has found in the past, impeachment is a political process, not a legal one. "Not justiciable" in their terminology.
The Founders spent a lot of time justifying the process for impeachment that they came up with, and why specifically they didn't want them tried in the Supreme Court. To imagine then that they'd include a backdoor where the Chief Justice can just throw out the case for any reason at all is ridiculous. You're literally arguing that a process the Constitution says requires two-thirds of senators can alternatively be decided by one person.