How to troubleshoot a water softener?
You have TWO tanks; the resin tank and the salt tank, you don't say which one has the odor in it and I can't see either from out here in SW AZ in my motorhome. Maybe if I went outside and looked..... I assume it is the salt tank because you don't mention having the control valve off the resin tank.
That nozzle, it is the injector and may have an injector throat too. And it will have a filter screen that will block up before the injector and the injector throat does.
Adding IO to the salt tank wastes IO and it doesn't do any good until it gets into the resin so I say mix it in water and pour it into the salt tank and do a manual regeneration is the best way to maximize its use. See my post to someone else here.
Yes there is a much smaller flow out the drain line during slow rinse/brine draw.
23 years in water treatment and well pumps, 13 years on the 'net helping others to help themselves.