Hi Dave,
You produced a very nice document! I can't compete with that with my poor pencil notations for personal use.
I think the description of what happens in step 2) of the sequence of operations is not fully correct. There might be some leakage which causes the voltage at C90 to drop during the next counts. But that's not intended. The trick in my opinion is that while charging C90 the voltage at C90 lags behind compared with the voltage at the base of Q90. The charging current (mainly depending on R74) also has to pass through R90. This causes a small voltage across R90. Hence at the moment of firing the voltage at C90 is slightly below the voltage at the base of Q90. That base voltage drops to the C90 voltage after charging has stopped. The setting of R90 determines the amount of voltage drop.
Albert
