The only thing I can think of where potassium is a problem is HYPP, a genetic problem in QH and Paints which carry Impressive blood lines. Since she's a rescue, I guess there are no papers for her. To be sure, there is a test for it available. Feed wise, the greatest problem would be alfalfa, which is high in potassium, and HYPP horses shouldn't have that.
On the second pic she doesn't look overweight at all, rather lacking muscle, and topline. Neck is definitely there, a slight crest also. For that I'd keep her low starch/ sugar, so grass hay, free choice, and the TC 3O%, and see from there. The TC 30% is a ration balancer, so she'll get all the nutrients she needs on top of what the hay supplies. The TC senior is a very nice feed, but I'd be very reluctant to give it to my IR horse.... just smells too sweet and is too dark.... it screams "molasses", which my horse can't have.