Using your analogy, which is wrong btw, but let's just say for the sake of argument that this is like the investigative stage of a criminal investigation...That being the case it is also the right of the accused in these cases NOT to cooperate with investigators too, and that is NOT obstruction....
You people are making the charge, now you have to prove it....
That might be true if it weren't for that pesky little law called Obstruction of Justice and Conspiracy. And you are right, the Accused does not have to cooperate but others do. And to instruct others not to cooperate and they follow that "Advice" makes them liable for the Obstruction of Justice and Conspiracy charges which can be easily proven. While Rump may not be bagged for that, himself, his merry band of criminals will be. And that is why so many are rolling over.
As the President, Rump gets a whole new set of rules. And, I know you think he doesn't have any rules, but he is barred by law to use his office for his own personal gain. That includes obstructing any investigation into him or his actions. He's protected against prosecution as long as he's President but what an Impeachment Trial is all about is to whether he remains President or not. If he is removed from office, all the charges can be or might be totaled up by the next President's Justice Department and Rump may be in serious trouble. Rump had better damn well pray that Biden doesn't become President in 2021. The way Rump has treated Biden, if I were Biden, Rump would be behind bars so fast and so long that you would have plenty of time to forget his name. What Rump doesn't understand is, be careful how you treat your possible replacement because you will be treated at least as unfairly as you treated them. It also WILL happen in 2024 as well so the day of reckoning is coming. We just don't know the exact date.