HH2; nav light Perspex, yes this would certainly soften, often thought to be due to extended reheat use.
As for high speed heat problems, a test flight from Leconfield (a gentleman of small stature) ‘recalled’ 93,000ft after a well timed zoom, but ran out of up elevator on the way down, and during the nose low recovery recorded M2.2(+?). The forward glass windshield was replaced as the view was slightly wavy.
IIRC the pitot probe also acquired a ‘tempered’ sheen aft of the heated section.
BAe reported that test flights achieved 700 KIAS at low altitude as part of the structural proof.
Re rudder / fin failure, wasn’t shockwave induced fatigue a contributor due to high speed in (very) close formation?
I recall being in a low level formation attempting M ~ 1 over The Irish Sea when the Irish ferry appeared over the horizon ….
Then there was the 4 ship which did similar (> M 1.0) near Papa Westray - with the Russian fleet in attendance.