Also, while this itself will not cause the IFRAME not to render, I immediately see that the IFRAME and the main document don't share a private domain suffix. You should know that Safari will not send cookies to the inner document in this case, so (for example) Pre-Fill will not work. You've done a great job visually integrating these other documents w/the forms into your layout, but what's the reason you aren't using embedded forms?
Fair point. There were a variety of business-related decisions that set us down the path of IFRAME rather than embed... But if we can't figure out how to load the forms consistently through IFRAME, we may have to course-correct. Though I certainly hope that's not that case!