Keith,
Interesting response from the AMA. One would think that the MAC range would be applicable to all airframes, both scratch built and ARF. Sounds like they are assuming the ARF is proven which may not be the case. If it was me, I'd move the pivot for what that's worth.
I applaud you for checking things out with a borescope. I've been meaning to pick up one of those myself. When checking the stabs or wings, you see something you don't like or are unsure about and want to bounce it off a structures AE, feel free to give me a call. You might also be able to easily get an x-ray. A couple of years back there was a big concern over F-20 stabs and pivot attachment points. One of the owners brought his stabs to an airport and got the TSA to x-ray them. I guess he went at a off-peak hour as they really knocked themselves out to help the guy and turned the effort into a training exercise. He got some great pics. Just a thought.
Jim