The water pump pieces can be so small that they get stuck in the head coolant passages causeing a hot spot that can crack a head.

if you can match up the pieces of the impeller found with the old unit, that's how you insure you have everything out...do I hear you right that you have 2 of the three pieces? thats a good start and maybe you will get lucky.

update
 

So since I last posted forever ago, I have flushed the cooling system twice, replacing the oil cooler o-rings, water pump, and thermostat. In the mean time I have also had to replace all of the intake boots and half of the clamps, and the oil dipstick tube to keep the car running. The top is now manual operation as well. But I have driven it nearly daily ever since the flush and still no oil in the coolant so that is a positive. Now I have a cold start stumble I can't figure out. It's usually only in the morning or if the AC is on when I start it. If it stumbles when I start it it goes away if I shut it down and restart it. I can't find any vacuum leaks so I am kinda stumped as to what it can be. I did remove and clean the throttle body and related intake components but all of the vacuum tubing was in order when it all went back together.