The speed of sound depends on temperature and pressure . . .
Nope - speed of sound in AIR in a 500 psi tank is only dependant on temperature. In the extreme ( vacuum ) with no air molecules speed of sound is of course zero. But we are not talking the extreme here but within the normal range of flight- and as such its strictly temperature
basic calculation speed of sound (m/s) = 331.5 + 0.60 T(°C) note only a relation to temperature - not ' pressure' or altitude withing the normal range. Check any table for speed of sound vs altitude and note that it is the temperature standard at altitude which controls
Which gets back to my oversimplified question - what other inputs control MCAS -- and how measured or compared to what standard versus actual ??