Troubling story and some questions

Hi Gang
Last week Minden had some excellent wave soaring. Off tow at 7.7k
msl and within minutes up to 18k. I decided to go up north skirting
Reno Intl. I had the transponder on and was monitoring Reno Approach.
My goal was to try and fly as fast as possible maintaining an altitude
very close to 18k by speeding up in lift and slowing down in light
lift or sink. This worked well for the first 35 miles where my IAS
(indicated air speed) ranged between 50 knots and 110 knots flying
never less than 17k. Then I got into some real lift and pointed the
nose down and noticed my Becker transponder registering 18.2k which
is, of course, a no no. I then glanced at the IAS. It read 138 knots
which is 165 knots TAS. (Every 1000 feet of altitude above sea level
results in an error of 1.5% in IAS.) So what to do? If I pulled the
spoilers at that speed, the shock might destroy them or the glider. So
I gently pulled the stick back and translated speed into altitude
going above 19k. At about 70 knots I pulled the spoilers and got
myself down to below 18k. I wonder if ATC caught that? Why was this so
bad? Well the VNE at sea level for the SparrowHawk is 123 knots and it
has been demonstrated that at 171 knots the wings come off. This
really gave me cause for concern. How quickly one can get into trouble
by not paying attention. In the future I will fly slower and use the
spoilers to compensate for excessive lift so as to maintain altitude.
This story raises some questions about VNE at various altitudes
which should be of interest to all of us glider pilots. I Googled
combinations of words such as "flutter altitude", "VNE altitude" and
"aircraft breakup altitude" to try and come up with information on
whether the flutter/breakup characteristics of an aircraft are less at
altitude than sea level at the same TAS. Intuitively it would seem so
but intuition may not work here. I found nothing useful. I know that
the World's ultimate high altitude motor glider the U2, which was
designed in the 50s, had much study done on it with regard to its
operating speed window of about 20mph (stall to breakup)at 80k feet
msl. There should now be declassified documents on those studies which
might answer my questions. I would appreciate any pointers anyone. If
I find anything useful I will summarize it on RAS.
Flying is often unforgiving of errors and I will definitely be more
vigilant after this wake up call.
Dave