[Steve Tinker]

Spinnaker, good catch, I'll bet you feel better now!
Do you think your (old) pump is original Porsche - I can't see any part numbers in the photo?
If its a Laso or a cheap knockoff, you are very lucky as the pump only had 50% of the blades pumping fluid....
When you went fishing for broken impeller bits, did you remove the thermostat to see if any of the debris was hooked up in the t 'stat housing - sometimes the 'stat collects them.
I hate to think where the impeller remains are located, but perhaps the finely ground dust adds up to the sum total of the missing impeller bits.
And to reverse flush the system, the 'stat should be removed anyway - it helps with the fluid flow.