@Janrain
I have no knowledge concerning SS2 and how it is designed to behave.
I've done a few handful of flights up to M2.3 in the phantom and up to 50.000 feet. This jet was getting the more stable the faster it flew. If i would have encountered such a rock'n roll motion in any flightphase I most probably would have aborted the flight. The problem with such non linear oscilations is, you never know how big the next one will be. And they must create some loads on those feather fins too.
I'm curious to see some footage of the fllight prior to the breakup, wether those oscilations had been present too.