It's really impossible to say, and that's part of what's so frustrating to the Iranian-American community today. I'd say it's more probable than not that the third scenario would occur at this moment, especially if the itinerary doesn't involve contact with the AA security theater at LHR. However, I'm just an armchair activist on these issues and can only offer an educated guess...

FWIW, my impression is that sometimes frontline officers are very suspicious of any contact with Iran (I haven't been but my wife is Iranian), but higher-level supervisors and analysts in DHS and/or its contractors consider Iran to be in a much lower risk bracket than Syria and Iraq, or even some of our "allies" in the Gulf region. It seems like it's travel to weak and failed states and conflict zones that is pushing risk scores through the roof right now, not repressive regimes with robust security services. In 20/20 hindsight aided by a recent FOIA disclosure, I don't think it's a coincidence that the Immigration Advisory Program lost interest in me after I finally revealed that I was travelling back and forth to IST to visit a Persian lover (and, thankfully, not a Syrian - that really seemed to be of interest in the inspection report!) Even if spotted, I'd like to think that a more seasoned CBP officer might choose to overlook an Iran stamp unless there has been some specific directive within CBP over the last 24 hours, especially at a POE with some experience in dealing with Iranians like LAX.

As for the motivations behind HR 158, it's good to read that I'm not the only one not buying the official line being reported in the media about counterterrorism! I have a feeling that there's a diplomatic cable out there with some interesting backstory. There's a lot of interests with access to Congress that would like to see the JCPOA fail, from US shale drillers to AIPAC. One of the limitations that the few Iranian-American advocacy groups like NIAC face is that they see cases like Rana's and tend to scream discrimination and Islamophobia, rather than recognizing themselves as part of a larger geopolitical tit-for-tat...