IMHO, prior to any impeachment hearings, his Constitutional qualification for occupancy of the office needs to be established. Impeachment is the prescribed remedy for removing somebody legitimately holding the office. I’m not sure O is entitled to that.
I agree the questions about his eligibility to even be in the impeachable office to begin with are important. This has come up before - impeaching him for everything he’s done there doesn’t at all endorse or establish any eligibility he has to be there. Any and all fraud involved in getting there could certainly be added to the list. I hope it is and that he’s called to answer questions on it publicly. This seems to me to be one of the best ways to do that.