I need help to understand a sniffer circuit schematic
C64,65 are probably to slow down the switching of V53, this is a slow protocol and nothing is to be gained by providing lots of high frequency gain. R91/C64 are a classic snubber network that adds loss at high frequency. 

I think there is a missing resistor between a supply rail and the gate of V8 as I see no way to pull this up in that schematic. 

When the opto is on, current flows in R87 sufficient to develop about 0.7V across the BE junction of V49, this pulls down the gate of V8 so as to limit the current in the opto to approximately .7V/825 ohms = ~1mA as long as the bus is above the 5.1V (V51) + opto coupler LED forward voltage, so maybe 6.5V or so? This assumes my missing resistor really is missing and not just missing something obvious.

There appears also to be some current sensing stuff over to the right that can also shut off the opto under some conditions I cannot be arsed to figure out.

Stupidly complicated for what it does!