I believe the mathematical analyses of recovery altitudes assumed a constant average nose down pitch rate (based on actual pitch rates generated for short periods from the FDR data) and a specific constant 'G' applied at a given speed on the pull-out. Correct me if I'm wrong.
This is correct for the entry and exit manoeuvres, but for by far the longest time the aircraft was assumed to be held at constant pitch attitude - in most cases 10 deg ND but other attitudes were looked at. The thinking was that attitude was about the only thing he could rely on and hold easily. 10 deg was a "for instance" value which might not have been too extreme to be believable, and was held for a long time just to see if it would have been effective. So your next point is OK - the sums did not depend on the ability to hold specific pitch rates over an extended period and weren't really any sort of function of FCS except insofar as the entry rate of pitch was concerned, and even there one could show that the entry rate did not significantly affect the outcome.