One thing that I don't think the House managers have ever discussed that they should have discussed.
There is a perfectly acceptable way to force Ukraine to investigate a crime that breaks US law: (1) The US Justice Department opens an investigation of the crime (2) The US Justice Department asks Ukraine to help with the investigation under Ukraine's existing treaty obligations. Ukraine has to comply.
When Ukraine was asked to investigate, they should have said "Have your justice department contact our justice department". The US should have made the request that way because that it the legitimate way to do it.
Schiff should know this. I know it because of testimony before his committee. Marie Yovanovitch pointed this out during her testimony before Schiff's committee. She almost said "Duh, you are suppose to do it this way." She did not say "Duh" but her facial expression, body language and demeanor when she said it had "Duh" written all over it.
The key is that Ukraine interference in the 2016 election would be a crime against the US. Joe Biden, as part if his official duties, extorting action from Ukraine for personal gain would be a crime against the US. Not sure if anyone has even alleged that Hunter Biden committed any crime under any law anywhere, so that one is harder to address this way I think. But Hunter Biden could have arguably have been investigated as part of a Joe Biden investigation.
At one point. Yermak did ask Volker for an official request from the US government to open an investigation. Volker asks someone in the state department if we had every made a request like this and they said "I sure as hell hope not". But if the US Justice Department was willing to open and investigation, they could have made an official request.