Yes you can, just note the blocks weigh 50lbs. Depending on your salt tank, can be harder to put block in.
I probably go through four or five blocks a year with nine grains of hardness for two people in the household.
Mine is a RainSoft water softener/filter. I would imagine any water softener that requires salt blocks.
It is the same, just solid piece. Check with your softener company and I expect they will recommend the solid salt.
You can mix any form of salt for you softener. Your softener requires brine water to rinse and recharge the media that performs the softening.
I am concerned about some sources of salt that folks make available to deer. Salt marketed for different purposes often has additives to make it flow better, bleached to make it a certain color, etc. It is certainly possible that these additives could be detrimental to deer and other wildlife. Therefore, I only use feed-grade salt. I personally use Trophy Rock, which has 60+ trace minerals.