Pete,
I believe the test house is correct. If there is more than one resonance in a frequency band (+/-10) then there is more than one thing in the system causing that resonance. The qualification testing is suppose to expose any feature that may cause failures therefore was designed to dwell...
About the best book I've seen is "Spacecraft Structures" by Carl Osgood. It was written in 1966 (but they did some impressive space projects in the 60s). It isn't easy to find, but it is worth getting. His book "Fatigue Design, 2ed" is great too.
I have to weight in here to say do NOT use any of these new forging materials without running your own tests to verify the material properties with actual parts. Size, shape, and many other factors will contribute to how strong and durable the actual parts will be. The real fatigue properites...
To answer thruthefence's question: it depends on the certification requirements for the aircraft and how the seats were certified. If the cushion has specific requirements in order to meet the dynamic seat requirements then that could be the case.
The same goes for the amount of flammability...
For a cheap and simple check if the parts not not too expensive; make a salt solution of water, partly cover the parts in a sealed container and put them in an over at 60 deg C for 48 hours (checking on them every few hours). I would do this with both your current parts and the potential new...