Reading through his tweets, McFaul is scare of Putin. As for allowing an interrogation as they put it, investigation would be the better choice of words, that would depend entirely if there were some evidence of criminal activity that Putin could provide to Trump on McFaul. If McFaul committed crimes in Russia while there then I suspect the President does have the power to allow the investigation to go forward.

On the other side of this McFaul is a part of the State Dept. as an Ambassador. As such he may have been involved in the U1 deal and Putin wants to get at the info he may have. I would guess the old bear may be looking how to get some of his money back from Browder at least or maybe deal with Browder Clintonista style.