I think we are in danger of confusion here - what exactly do you mean by a 'high-speed stall'? The 'Shock stall' referred to above is not the 'high-speed stall'. The HSS is when the angle of attack of the wing reaches stalling angle at a speed above the straight and level stall speed. This occurs when the aircraft is being 'accelerated' ie 'g' is being pulled, eg in a steep turn or aeros, because the 'weight' of the aircraft is increased by the 'g' thus raising the stall speed.
IF this is what you are asking about, my answers are:
Q1 Low altitude (more 'g' available)
Q2 High and High ( " " )
NB Different answers for a 'shock stall'
