I'm not sure what's in this. Do you have a high fill CH such that you have a positive CSI?
This is tough to answer. Not being used really takes a while to get there. After a night of heavy use (3-4 people for hours at 100 °F) I can see increased chlorine demand for well over a week as residual bather waste is slowly broken down. Usually we use the tub again before it returns to a "baseline" number.
I think the best I recall is from our previous tub, an inflatable Intex. Due to the fact that it couldn't maintain temp on the days when it's best to use (cool fall or calm winter nights) it never got near the usage our 6 person Cal Spas does. No long nights with multiple people drinking for hours. On that I think the lowest drop I remember is somewhere around 20% in 24 hrs (10 ppm to 8 ppm), but it's been a couple years so my memory is foggy.
On my Cal Spas with a Saltron Mini SWCG I don't usually measure drop, the SWCG maintains chlorine when not being used so I just check and add the first few days after a heavy use, then check every few days to a week when not used to see how the SWCG is keeping up.
Honestly I may just leave it for the moment and see what happens. You mentioned borates and they may be useful. I haven't used them in our Cal Spas yet, I tried them in our Intex one but that didn't have much aeration, I was going for the "feel" some talk about with borates. Didn't notice that, so they'd be purely to slow pH swings in the Cal Spa.
Yes, this is an excellent point, and why I asked what you were adding. Dichlor is acidic and will lower pH. Honestly I've personally stopped using dichlor then bleach method. I have straight solid CYA for my pool, and I can add this and it doesn't affect pH much at all. Unlike a pool CYA dissolved in a hot tub pretty fast (hours), so I don't use a sock and just dump it in near the filter so the water action helps dissolve it.