deciphering schematic and diagnosing fault in cine camera circuit
Much trial and error and frying of diodes later (best to draw a veil), this is what I know:


Yes, PW is a photoresistor (thanks to venny).
No, the arcs do not represent variable resistors, they just show that the switches are rotary.
Neither the switches nor any of the solid-state components were faulty.
It is true that the photoresistor was not the problem (it is indeed cut out of the circuit with the switch in the battery-test position).
Not sure why the labels D-D and S-S are doubled up but, as far as I can see, they are just arbitrary names for those points in the circuit.
The circuit is essentially a bridge (at least, it is in automatic exposure mode). The arrow-in-a-circle is a lovely little gadget connected across the bridge. The gadget opens and closes the camera iris, depending on whether one side of the bridge (B) is at a positive or negative potential relative to the other side (S-S). Opening and closing the iris changes the amount of light reaching PW, hence its resistance, so that the bridge dynamically balances itself.


When I unscrewed the gadget from the camera chassis, it seemed to work properly, but when I put it back in it just slammed fully open like before. The gadget is made with moving and static coils arranged very snugly in a small metal barrel case. I eventually figured out that one of the static coils was touching the case, causing S-S to short to ground when the gadget was screwed on to the chassis. To my surprise I was able to prise the cap off the barrel, stick a bit of insulating tape in there, and put it all back together without actually destroying it. I now have a working camera (except for the remaining problem of recalibrating the exposure - adjust variable resistor P1A - without a known intensity light source).