I have had stems with a little supplement bolt, beside the main binding bolt, that was clearly intended to expand the gap enough to facilitate removal.
There wasn't one obvious on this unit but, holy smokes. as I was reading your suggestions I backed off the bolt to see if - who know! - maybe there was some nifty reverse threading on the big bolt... But Now, it just came right out.

Only to reveal TaDa! a little bolt hidden inside the barrel of the larger bolt.
How cool is that! Those crazy Italians (this is ITM, who of course also made the insane seatpost clamp that had me baffled for a full year.)

Lesson? (there always seems to be a lesson.) When the answer seems reallllly complicated, (or when force seems necessary) it probably isn't.

Peter