Sorry to hear that, but we only based it on the information initially provided. For example, you only provided a mockup design and ConvertPlus, so we thought you're referring to info bar. And then I just suggested the widget area since it's much simpler if its main purpose is collapsible info bar.
But now all the requirement is clear, I'm afraid to say it's not possible. The theme has no element or existing feature for the collapsible section which is why our last recommendation is using a 3rd party plugin.
And yes, it's completely impossible for ConvertPlus since it's not made for that purpose. It's either use a 3rd party feature or do custom development which I recommend contacting a developer to fully create a functional expandable content.
Thanks for understanding.