Um, I'd agree with the "even-handed trial" part if there were in fact actual high crimes and misdemeanors in dispute.
Not seeing any in the current set of Articles. In an actual criminal trial, a judge would throw them out. Trials are not designed to be "fishing expeditions." The purpose is for the Prosecution to PRESENT evidence, not try to find some by putting people on the stand hoping they might be able to provide it.
Meanwhile, as other people have argued, this is not an "actual" trial. This is true. The "Investigators" were the House, and before they voted they had a chance to try to get evidence. They had complete control of the process, which they then used to demonstrate absolute political bias.
The trial according to the Constitution, is conducted by the Senate. As per the same Article of the Constitution, they have complete control over the process. They want the process to mirror precedent under the Clinton example, but if the House keeps making demands, the Senate can change the rules.
Meanwhile, it's a bit late to be crying for evenhanded, when based on the lack of any real evidence, IMO no "trial" has to occur. The Senate can listen to the non-evidence provided by the House Managers, Trump (if he is wisely advised) can just sit it out, and he'd be acquitted fair and square.