Engine shutdown?
I'm late catching up to this thread, but 7 pages in, I'm still wondering - what makes you all think that the Captain shut down the engine? I'm not familiar with the Dash 8, but I have flown other P&W turboprops, and as far as I know they are all free-spool power stages. On many (all?) you can feather the propeller completely in flight without shutting down the engine, leaving the engine at idle. I know that the passengers were told "as a precaution we are going to shut down the left engine", but that's what I would have said on the PA too, even if I was leaving the engine running and just feathering the prop. It's too hard to explain it any other way when the propeller is quite clearly stationary.
Meanwhile, since the hit was the "lucky shot" right on the spinner, I imagine the only symptom on the flight deck was a "thump" on the climb-out. With all the T's and P's in the green, both engines turning just fine, of course you keep going - fly the SID, get up into the cruise. Then (20 minutes into the flight) the F.O. went back to see if there was anything to see, returned and said "we've lost a huge chunk of the spinner". They decide to feather the prop to limit any further damage, but carry on because there's nothing obviously broken - and if my guess is right, both engines are still running. If the worst comes to happen, they can always unfeather the one with the broken spinner and get back to two. No relight required.
So - anyone here flown a Dash 8? Can you feather the prop with the engine running?