Hi,
DHC-8-311 Captain and I'll have a go at explaining why you don't shut the engine down.
As you say, Hydraulic protection starts at 1236rpm(103%Np) and Pneumatic at 1308rpm(109%Np). Aerodynamic loads drive prop to fine or low angles and oil pressure(780psi) drive it to coarse or high angles.
The system achives Constant Speed by;
1. Metering oil pressure to one side of the pitch change mechanism in the hub- drives to fine, and this is opposed by;
2. Constant oil pressure(780psi)on the opposite side to drive the prop to coarse.
If you remove oil fom the hub(ie only one side), the 780psi drives the prop to feather. However, if the entire PCU loses oil pressure, then a pitch-lock mechanism activates to stop the aero loads increasing blade angles more than 1 degree and prop rpm from increasing by more than 2% from the point of oil pressure loss.
Now, if you have lost control of the prop and you shut down the engine, you end up with a 13 foot diameter disc on one side and large increases in Vmca. This is why we, as per chechlist, fly as low as practicable(thicker air therefore more drag on the prop) and as slow as practicable.
I hope that makes it a little clearer for you.