Are you sure they have supercritical wings?
I cannot see why the 767 would not have a tendency to Mach tuck? I highly doubt that any of them incorporate such wing designs...
What you refer to as limiting speed is the so-called critical Mach number; the speed where local flow (usually over the wing(s)) reach mach 1.0. When the air decelerates to below M1.0 it does so through a shock wave. The air passing through this shock wave could suddenly separate (detach from the wing) as a result from the great increase in pressure & decrease in speed as it passes through the SW.
When the plugs melt, the Nitrogen inside the tyres is released, and this cools the brakes = no fire! (I think)