The problem with holding someone back is that the city has the ability to raise a shield, so if problems happen there's a good chance whoever is left on watch won't be able to get in. You're right that they should have someone ready to back up the infiltration team, though.

So how about this--Sol and Wally go in alone at first, but someone attached to the diplomat team acts as their handler to keep track of them. That way if anything happens the handler can run interference and the diplomats can claim they themselves were infiltrated. It keeps the teams politically separate and gives the diplomats plausible deniability.

By 'someone' I do happen to be thinking 'Kaldur', if that's what you'd like ... :3