He could not be called to testify.
He could choose to testify but he wouldn’t.
Seperation of powers.
The House is entirely alone in the proceeding. The defenders of Obama in the House would be the House Democrat leadership and their chosen henchmen.
Obama won’t be impeached. Obama won’t be prosecuted. So it’s a moot point.
Impeachment maybe not.. but prosecution I think is still possible. I wouldn’t rule that out because we do not yet know all of the evidence.
There are emails, audio recordings and videos that the public has not yet seen.
If he was impeached after he left office he could be compelled to testify, and he would not be able to claim executive privilege on any of the government records.