I've done a bit more testing. I set the System every x seconds to check and see if there was any Ajax.LastData. And, the .php script is sending data back to the Ajax Plugin as the LastData is exactly what I'm echo'ing out of the php script. But the Ajax plugin is still not detecting an on Completed or on Progress event. I've tried changing the headers on the .php file and nothing.
So, data is flowing back and forth, the Ajax plugin simply can't tell if it's done or not.
I'll take ANY ideas at this point.
Since the IAP isn't working with Chrome Web I'm having to look at PayPal integration and in order to check registrations, I'm gonna need to run a .php script. Since there is no simple GET or POST with C2 and since the browser opens everything in another window, Ajax is my only hope.
If I do succeed, I'll post instruction on how I managed this.