Hattori Heizo would be my guess for RUM too. Most, if not all, secret organisations in history (or at least in movies lol) have high ranking members who infiltrated into the police, the Cabinet, or both. A powerful organisation wouldn't be powerful without this element - imagine the resources they could reap! And how else would they be able to be under the radar for so long?
I don't see the need to *look* like all 3 of the descriptions Haibara mentioned. It could just be one person who don't even fit any of these descriptions, so that he could be "invisible" even from within the organisation. He could have a group of messengers who don't look anything like him, to confuse the parties he work with. He might be a little young to be second-in-command, but who knows? He could be related to the Boss and he's the successor.