Hey, not sure if you still need help with this, trying to clean up some old questions. I don’t really have an answer for the way you are attempting to do this, but I do have a suggestion for an easier way to do this. I try to do things in the simplest way so that I can get it done faster.
If I were to attempt to do this I would load an iframe into the modal window. I’d create a special template that had only what was needed to make the page and acf_form() work.
Then you can use acf/pre_save_post or even acf/save_post hooks to do whatever you want to do with the posted data, including put it somewhere else.
