I was almost finished my post when yours arrived. I was reporting my results of testing that showed that, like your lead developer, HTTP_REFERER has the src= URL from the iframe.
But, I still see the correct value in HTTP_REFERER in child2 and child3. Both on my test system and with the links to your site.
I'll be away for a few hours now, but when I get back, I'll do more testing and then proceed to a Beta version of the plugin, which I'll notify you of, so you can try it out.
Thanks again for your patience. Sorry that this has taken so long, but I have no experience with iframes with this plugin, and the possibilities just never occurred to me. Until now.