Going back to this question of Stan and his decision in the parking garage. A couple things which haven't been touched upon yet.
For one, from the moment Philip stated that they were going to drive away and hopefully they will be allowed to (by Stan) to the point where they had actually pulled out of the parking garage, the torment on Stan's face was plainly evident. You could tell he was wrestling mightily with this decision the entire time, even as he let them go and drive on past him. Fantastic acting job by Noah Emmerich here.
Secondly, given his friendship with Philip and the degree of vulnerable honesty (to a point) that Philip was finally expressing to him coupled with the fact that along with this new honesty, both Philip and Elizabeth were still manipulating him at the same time (denying any killings or at least the most recent killings of the Russian couple). Also emphasizing how Philip had been out of the spy business for years now. Which not only helped Philips argument, it also served to help deflect some of the attention off of Elizabeth and her ongoing involvement for the moment.
Then when you toss in the subject of Henry and the issue with P&E's claim of being the only ones who could now stop the Gorbachev overthrow, to my way of thinking, I do think it is entirely plausible that a real human being confronted with such a scenario would sometimes make the very same decision as Stan did. No matter how well-trained or otherwise competent the law enforcement agent.
But then again, I could just be a sucker here, seduced by a well-executed television drama.