Regarding the omega horseshine, one cup weighs 3.5oz. TSC carries it, 40$ for 20lbs, so it'll last a while. Start out slow and I think more than 1/2 cup daily would be okay for her. Split between two meals.
Now with the salt, she should be drinking more anyway, but when it's really nasty and cold outside mine always appreciated tepid water, would drink two buckets right away. I did have a colic case in winter due to cold water. Senior horse, too.
When I had my horses NOT at home, I used to measure out the supplements and put them in little sandwichbags, took them to the barn so if somebody else was feeding all they had to do is just dump it in the buckets.