Kris, I have about the same size pool and I filled from the treated house water and it saved me a ton of money/time/hassle!!! I'm on a well also, with high iron and lots of other 'fun' stuff (sulfur, etc, etc). It only took about 9 hrs to fill with a garden hose, so if it's possible for you, I would HIGHLY recommend it (as long as your house/treated water is indeed better).

I never checked into the prefilter, but from what I've read here, no one has named one that really works (that you can buy). There have been a few 'homemade' filters, but not sure it's worth it in your case (you have a smaller pool and filtered water already).

Yes, the Taylor K-2006 is more accurate for testing high shock levels and overall just a better tool to enable you to take control of your pool!!! There are several differences in the testing and the Taylor is really, really worth it!

If I were you I would forget about the sequestrates & other 'things', and just refill... But, that's just my opinion... I'm of the mindset that less is more when it comes to putting stuff in my water!!!