Though Schiff is only being asked to do closed door testimony, he also may qualify under #9 as one of the others who aided the whistleblower in preparation of his complaint. 3 out of 4 ain't too bad. None of those three will be required to testify anyway before House Committees. The Senate Trial will be able to explore their testimony. Hey maybe Joe will be invited back to say "hey".
Here's the letter requesting witnesses: www.scribd.com