Reduced AGC on the mixer for the HF bands is an interesting aspect of the Chapman S6BS that as you say is not shown in the schematic nor mentioned in the brochures. I am not so surprised though, as that model was CTC’s pièce-de-résistance with much attention to detail, and which, with some changes although not to the core design, stayed in production from 1953 to c.1969. The AGC IF amplifier sidechain allowed the application of full AGC bias to the final IF stage without getting into modulation rise distortion, and also avoided differential distortion. In that case the EBF80 did not need neutralization, perhaps because the demodulator diode anode was fed from the 2nd IF stage whereas the pentode grid was fed from the AGC sidechain IF stage.
Re the Quad AMII, the detailed circuit description was also included in the sales brochure. This was something of a “textbook” design from John Collinson. It had fractional AGC (about a quarter) on the IF stage to minimize modulation rise distortion, but given that it had an RF stage, there was room to do this without unduly compromising the AGC range. I suspect that the small case size mitigated against having a large valve count for heat build-up reasons, hence the single high-gain (and neutralized) IF stage rather than two lower gain stages. Still, there was more than enough gain for the primary purpose of high-quality MF band listening. Here is the AMII EBF89 neutralization circuit:
Basically, C32 neutralizes the EBF89 pentode anode-to-grid capacitance, and C37 neutralizes the diode anode-to-pentode grid capacitance. It follows the AWV Radiotronics circuit, which was also shown by Langford-Smith.
As kalee20 has said, the designer of a simple MF/LF receiver with just a mixer and a single IF amplifier was between a rock and a hard place when it came to AGC, and the usual approach seems to have been to apply full AGC bias to both stages. Sliding screen bias on the IF stage evidently helped somewhat in reducing modulation rise distortion. A high-gain IF stage would be preferable, but then this would likely require neutralization, perhaps not welcome when cost reduction was the objective. Degeneration by omitting the IF stage cathode resistor bypass capacitor was another way of reducing modulation rise, and it saved a capacitor, but that doesn’t seem to have been much used; the loss of gain might have been a concern.
Cheers,