I think it's a good idea to either keep her blanketed all the time, means a lighter one for the day and a heavier one for the night, or leave them off all the time and let her grow fur. Come spring you can leave off the blanket in the warmest hours of the day.
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.