For about $20 you can buy a regular housing for a 10" x 2.5" cartridge with an off/on valve in head that includes a pressure relief valve button. For a few more bucks you can get that in a clear sump so you can see the water inside the housing. That type saves on the space needed to install it and the time it takes to install it and you don't need additional shut off valves.
To disinfect the water heater, you fill the housing 1/2-3/4 full of bleach and then run hot water into a glass at the kitchen sink and bending over to get your nose on the rim of the glass, smell for bleach/chlorine and when you smell it shut off the water. If after doing that for like 10 minutes and not smelling bleach, you probably need more bleach in your filter housing.
If it were me, I'd turn the heat up on the heater to 140f for an hour and in the future repeat as needed every few weeks/months etc. but....
It should be installed on the outlet of the softener. If you want to sanitize the softener use a 1/4 cup non scented regular bleach in a gallon of water and pour it into the water in the salt tank. Let it set for an hour and then do a manual regeneration and don't use water until the regeneration is finished. Doing the regeneration on your way to bed is best.