Sorry you haven't gotten responses to this yet--I've spent quite a bit of time on the forum in the last few days and this is the first time I've seen this post--don't know why. Anyway...
The only difference in the R 0871 and r0871c is the size of the bottle.
I'm not sure why your alk doesn't turn red--but I would count the drops until you get the last permanent color change and call it that level. Regarding CYA, other than biodegration by algae or some bacteria, the only explanation that I could offer for lowering CYA is replacement of water--either splashout, large amount of vac'ing to waste, or backwashing the filter. You can backwash out an immense amount of water with a sand filter.

Regarding the polyquat, I don't use it in my pool, but also have never had to conquer mustard algae either. My guess would be that it would be a good thing to use as a secondary defense to chlorine, but I'll let Pooldoc, Waterbear, chem geek, or some of the others chime in with a more informed opinion.