Yes, the LDRs are very sensitive to even the tiniest amount of light.
So by that test we now know that the high LDR is not shorted and it is working, at least on some level. Just to clarify, when you cut the high optocoupler open, did you see that the LED was pulsing at any time (or at any knob setting), or was it just staying on all the time? Because the pedal is behaving like it’s just staying on and never flashing, at least according to the symptoms that I’m aware of. If this is the case then there could be a problem with the mix pot. Please ensure that the center pin of this pot is scrolling from close to 0 volts to close to 5 volts as the knob is turned from one end to the other. Otherwise, there could a problem with Q1…ensure it’s in the correct orientation and soldered correctly, without any shorts.
-Brach