Hi,
"3. There is a radar track at the right time and place that has no irregularity worth commenting upon (other than the loss of Mode C). "
The radar track reached 14900ft and there is no mention that he used oxygen.
If he did not, a loss of consciousness is quite possible.
With a climb rate of 300ft/min, the climb from 12000ft to 14900ft would have brought him over the legal limit for nearly 10 minutes.
Joe