I couldn't find anything in my books on reasons for the angles. Perhaps the 30 degree opening is necessary for faster deployment on the ground? (especially if the autospeedbrake isn't working). Or perhaps the 30 degree deployment is best for loading up the wing progressively?
I wouldn't worry about minor angle differences. The books are full of these minor angle differences. The mechanical/hydraulic setup probably has a degree (or two) of hysteresis. Temperature variations will also affect angles. I don't think the system is set up by Engineering using angles, but, I suspect, by fitting rig pins to get the correct positions for adjustment.
The interlock valve failure scenario is interesting, but there are probably a few safeguards. e.g. the valve is spring loaded to the flight position. i.e. if the cable from the gear breaks, the valve will be held in the closed position. The valve has a proximity sensor and a hydraulic pressure switch which may offer warnings in a timely manner. Signals are sent to the PSEU and the SPEEDBRAKES EXTENDED light.
Cheers
NSEU