Well the MHD Extractor would probably not be used at all in cruise flight. I got the general inspiration to use such a device from this paper:
The idea was to use the Extractor for high mach numbers when the inlet gets more and more inefficient due to high total pressure losses. I'm not at all sure if this could be outbalanced using the MHD Extractor. The paper above speaks of a reduction of kinetic energy of the airflow by 40%, whith 20% going into generated electricity and 20% into heating the airflow. Also according to the paper the MHD Extractor would have a total pressure loss of 70% (!). So my idea was to not use it until the total pressure loss of the inlet rises above 70%. What i don't know is, what the total pressure loss of the MHD Extractor would be at higher mach numbers. The simulations in the paper where done at Mach 3 i think.
Regarding the Heat Exchanger.. yes i guess directly using the SABRE heat exchanger to dump heat into an airstream that was slowed down from M4-5 would not be possible. I just took the general figures for how much surface area per square meter might be possible and then used the thermal wizard () to calculate some heat transfer coefficients, saw the huge numbers and decided this might well be doable.