2. Heater core can be flushed with a garden hose. Motor can also to some extent...or simply drained via the drains in the side of the block.
3. If you're gonna keep after it and replace every 2 or 3 years, plain ol' green coolant works just fine. Some like to toss a bottle or two of Redline Water Wetter. It's good stuff. If you want a long life alternative to glycol & water, I'm a big believer in Evans: http://www.evanscooling.com/ I know nothing about the Royal Purple product you ask about.
4. I think OE spec for t-stat was 195? Many run cooler than stock these days...like a 180 maybe. Why? I dunno. For some reason people seem to have a hard time accepting that these machines run perfectly fine long lives even if they run at 200 or 210 all the time. Frankly I'm not sure what is "best". Certainly your pressurized system can safely run hotter than the factory open system, so stock 195 is likely perfectly fine. Other school of thought is running a cooler t-stat helps keep underhood temps cooler, allowing cooler intake air and more efficient combustion. Lotsa theories, not sure what's been proven. If it was running good before, you can pull the t-stat out and read it to know what was in there and copy it. The temp rating will be stamped on it.
While your pump is off, pay attention to vane/backer plate clearance and make sure it's nice and close. This does more for cooling system efficiency than anything else you can do on a Pontiac: http://www.wallaceracing.com/water-pump-mods.php
