Eddie, a lot depends on what the failure was and whether or not the core is still rotating.
With an engine that is shut down and the plane flying along you still get airflow through the engine and you'll see it on the Ng.
Now, since the Ng doesn't stop completely (unless say a bearing fails) the oil pump is still driven and sending oil to the primary governor, which boosts the pressure and the prop keeps spinning.
On the ground if you shut a PT6 down and don't feather it, they'll spin for a long time, stop, and as the Ng has also stopped the blades will feather themselves.
The need for the "separate system" you mention (the auto feather) is not because you cannot feather the prop (there's a lever for that), but if you go back several posts it is due specifically for Vmca certification criteria on specific King Airs.
In short the 4-bladers require autofeather because if they windmill they are VERY draggy. The auto feather system will feather it quicker than relying on the pilot to do it. But, you can still manually feather if the auto system fails (which I've never heard about, it's fairly simple and quite robust).