Is the iridescent part metallic? It should be, if it isn’t. Your specular highlight doesn’t look right which is why I ask.
This effect is a bit annoying to do with a roughness/metallic setup (like Unreal uses) because we can only change the specular color by setting the metalness to 1 and adjusting the base color. You can use the clearcoat shader if these cards normally have an additional clear coat over them (I imagine they must? Or they’d be pretty delicate? Not sure)
Honestly this seems like a reasonable approach and probably closest to what you’re going to be able to get without a lot of work.
Another option would be to just create a cubemap with the highlights in them and sample it from the reflection vector of the cards surface blending it over the existing iridescent effect might be convincing enough.
Me either 
Can’t help you on that road I’m afraid
You can use a stencil mask or alternatively you can just use a transparent plane that hovers above the region you want the post process applied to. It’s used a lot to give a post process effect that only appears behind the plane (depth fog for water is usually the most common example.) Both of these will require you to recreate the chromatic aberration effect though