Bit-- the answer to your first question is, whatever the surface, I look at all the horses. What usually happens is that when grass horses run on dirt, they don't run in their usual grass figure range-- you can't force them all into a 5 (or whatever) point spread when the gap from first to last is 15 points. It would be obviously wrong to assume that the grass horses were running to their figures and the dirt horses were all running 10 point tops, so as a practical matter you end up using the dirt horses, and giving the grass horses non-efforts.
Same thing happens in reverse for grass races.

In the case of Polytrack, more grass horses seem to run to their numbers, so they are more useful in working out the variant. Also, a factor I mentioned the other day makes it very easy-- where small fields make it tough to do some tracks (small sample size), the huge fields at Turfway make it very simple.

As for the other question-- I posted about this after a week or so of the meet, and the answer is that the track changes speed not only day to day but throughout the course of the card. Beyer wrote the same thing in the DRF, and we both agree that it is a good thing, too. Don't know why, but I suspect it could be temperature or humidity-- doesn't really matter, as long as you know it's happening.