The key to any successful RTLS is energy management. The reason for the dangerous PPA maneuver in a nominal RTLS is to bleed off the excessive velocity prior to ET sep and gliding. This must be done within the proper boundary conditions to succeed. In simple terms they must use the SSME to slow down. The problem facing the Challenger pilots was similar but if they took no action the STS stack without the SRB’s would naturally lose velocity. The situation at 74 seconds requires the remaning 51L stack to bleed off excess velocity and get in the correct configuration for post MECO coast and ET sep while not getting too far away from KSC.
So the answer to the first question is that the choice of MECO after SRB sep at 72 seconds is not critical, so long as it is not delayed too long. The vehicle in post SRB sep configuration is doing what you want it to do, lose velocity. Without the SRB thrust the stack is losing velocity at the rate of 10 feet per second even with all three engines running. My very rough calculation with a starting velocity of 2900 feet per second indicate that the shuttle will still be gaining 3800 ft/sec in altitude and moving 1962 feet downrange every second.
So after successful SRB sep give Dick and Mike 15 seconds to regain situational awareness. That puts them at 90,000 feet and 15 miles downrange. Velocity has dropped to 2618 feet per second.
It is at this point seat of the pants pilot skills come into play. Dick Scobee was a Shuttle Carrier Aircraft flight instructor and was familiar with the out of nominal release conditions for the Shuttle and the SCA. He would have been the best astronaut in the program to make this off nominal ET sep decision.
The best move is to increase the loss of velocity quickly so that you can execute the ET sep and start the glide back toward KSC. The most obvious way to bleed off velocity is to do a MECO. With no thrust from the SSME and a 1.6 million pound vehicle, the stack will rapidly lose energy. First, step would be to roll the shuttle into a “shuttle up position” MECO would occur at 95 seconds, at 2240 feet per second, altitude 111,000 feet, 18 miles downrange. The RTLS FSW would take over and control the ET sep process At 1300 feet per second the ET sep would occur. My calculation indicates that this would occur at 18 seconds after MECO or MET 113 seconds. Altitude would be 138,000 feet and 26 miles downrange.
At this point the Challenger is high and pointed away from the KSC landing strip. I believe the Gliding RTLS FSW would be able to handle the situation from MECO onward. This would be a “nominal” RTLS once MECO is set.