I also think the DNI and NSA's testimony is relevant as well. They weren't in the room but they may very well have been asked to do something illegal with regards to the Russia investigation.
What if Trump ordered everyone in the executive branch to stop investigating Russia? Would that be breaking the law? I think it'd be pretty ****ty. But is it illegal? The former president ordered the investigation. The current president could legally stop it. But politically that would be suicide.