Help troubleshooting a TS9 stuck on "bypass" mode?

[92Volts]

Sorry for the double post but I'm not really sure if Solid State or this Effects forum is the right section. I was hoping somebody might have some ideas on a broken TS9 Tubescreamer-- or maybe help me understand more about this circuit.

The problem is the signal is always clean. The LED does switch on/off with the button, but the sound doesn't change.

I'm learning from this site: https://www.electrosmash.com/tube-screamer-analysis
Schematic for this part of the circuit: https://www.electrosmash.com/images/tech/tube-screamer/tube-screamer-jfet-bypass-switch.png

To allow overdrive signal, Q2 gate is biased high through D3, to shut out overdrive signal Q2 gate is biased low through D3. At any point the opposite should be true for Q4 which controls clean signal and is biased high/low through D4.

My measurements show the bottom ends of D3 and D4 (down by C11 and C12) are getting variable voltage of ~30mV up to ~7v, and "flip flop" when the switch is activated. But only one of the FETs gets gate voltage that changes while the other stays down at 30mV to 90mV

I have 2 questions (besides "how do I fix it")--

How the heck does this bias scheme work? These diodes look like they should only pull the gate down. How does the gate get pushed up when the diodes are reverse-biased and should block current?

If only the clean channel switches properly, shouldn't it switch off and kill the sound instead of staying clean when switched? If neither was switching wouldn't it give no sound?

And of course-- how do I fix it? I'm suspecting either dead diodes or transistors but I'm having a hard time understanding how or what went wrong. If one of these diodes was dead could I substitute a different one or is the behavior of it critical to the circuit?

Thanks in advance!