I think the 15k, 0.01uF and 1uF network at the positive input is acting moreso as an AC voltage divider than a filter. At those values, you would be attenuating pretty much all of the audible spectrum. This is usually necessary with OTAs because they have a very limited linear range on the input (in the 10s of mV). They don't want to mess with the DC bias balance across the two inputs, so they reduce the signal level through that cap to prevent mismatching the DC bias between the inputs (if they just used the 15k resistor, it would drop the DC voltage at the positive input.
Think I've always analysed this circuit a little differently. I see the transistors in the envelope filter as switches if you like, when no signal is present they're closed and there is no voltage drop over the 150K resistor at their collectors, meaning there is a voltage at the base of the Transistor providing bias current to the OTA. Whereas when there is signal present at the bases of the transistors in the envelope filter, they're 'open' and there are pathways to ground, so current flows through the 150K resistor meaning that there is now a voltage drop and the transistor providing bias current to the OTA 'shuts off.' So when there is no signal at the envelope detector there is bias current being sent to The OTA to amplify, however as more signal is present at the envelope detecter there is less bias current being sent back to The OTA to amplify. Open to discussions of course 🤘
Correct. The other important thing to note is that, since the diodes are positive rectifying the signal, the output of the differential transistor pair at the collectors is actually a negative rectified signal (because they invert the polarity in this configuration). The transistor above that acts as a buffer and current source for the OTA's current-controlled gain input pin (non-inverting) so it drops the control voltage/current when the signal gets overall louder. There's also a cap there that filters ripple out of the control signal and seems to depend on the output impedance of those two transistors to do its job.
