1. I would think between 20 and 50. Then give it time. Don't forget to flush toilets, run the dishwasher a bit, do all hose bibs. The incoming chlorine will be higher, but I will stop the flow when I am at that 20-50 level to minimize putting chlorine into the septic.
Make sure you have your softener not do a regen while the big chlorine levels are present. Maybe having the bypass in bypass will be sufficient and an attempt to regen won't actually do anything. I expect to have my controller not make the attempt.
I will put my softener on bypass during most of the process, but I will pass some chlorinated water through the system when the level gets lower... maybe 10 ppm. I plan to put about a tablespoon of bleach in my brine tank, and stir that around early in the process. I plan to do a regen while there is still some chlorine remaining in the system. I expect to put 200 ppm into the casing, but do some things after much of the chlorine has been remove.
Because I cannot drop well chlorinating pellets past my pump to get to the bottom, I will use a much larger flooding volume to move chlorine solution past the pump to the bottom.
Run some chlorinated water through the shower as well as tub. Hot and cold.
If you have an RO system, that takes special action.
I will shut off my supply to the WH before adding chlorine. That can be a source of water for flushing toilets with a bucket. I will put my WH into vacation mode and drain my WH of fresh water before turning the water heater water back on, so the WH and hot pipes can get a good dose.
Take steps to minimize chlorine into the septic tank as best as you can compatible with the sanitizing purpose.
2. That is not a common or normal thing to do. If you did that, you would have to put a lot of work into getting the level right. You would want between 0.25 and 2 ppm. Measuring that would a lot of work and require tuning. It would also require low range strips. I would not do it myself, unless I had some special situation and I wanted to experiment.
