Healed—to be honest if you were at the end of your session I don’t see what difference two minutes would have made to the other client; the receptionist could have gotten them to hang on for that length of time. I think your feelings are completely legitimate. Endings of sessions can be even more important than the rest of the session.
As for emailing him, he’s just shown you in session that for him the squeaky wheel gets the grease, so I don’t think you should feel bad about all the emails. I do think you have the right to a response, just like that client apparently had a right to interrupt your session. Would a phone message get him to respond via phone or email?