CAA, FAA
You are doing well to get a response from the UK CAA. I have had my application in since Septermber and they dont respond to any of my emails. Even their complaints line dont respond. And if you ring, they arent at their desk and they dont call back.
Re the piggy back licence why not get a stand alone FAA licence. You can get condensed courses (I am based in Daytona) of 2-3 weeks , its alot of fun and you can rent a c172 with a gns430 for $100/hr wet !!. Once I got hooked into the USA system I gave up flying in the UK and finally moved here!.Night flying here is awesome with some many airports available, many with pilot controlled lighting. Flying up Daytona Beach at 500' is a blast.
The schools will help you sort out the TSA etc.