Feed: She should supply her own feed at her cost. Why are you going to get her mare's feed? Don't discount your board, I fear you'll set a precedent for other boarders wanting to do their own thing to 'save' 50 bucks or whatever. You have to store it separately and accommodate her, so she can go get it.
Barn Chores: Tell her you want to revisit what's expected, maybe post/give her a checklist to ensure your version of the barn is 'done' , is clear. Tell her you are worried that you two are on different pages and you need to be on the same one if you are going to be happy, and you want her to know what 'done' looks like from your viewpoint. If she puffs up, just tell her I'm sorry that this isn't working, but it's my barn and I need to be happy with how it looks, and this isn't working for me.
Ulcers: Not your horse, not your problem.
Fat: See ulcers. If she wants to feed the mare more hay, above and beyond what you offer, she needs to pay for that.
if you boarded with me, could I dictate that your horse go to ___ for an MRI? No, I didn't think so. Separate the forest from the trees, it'll get a lot plainer what you really need to worry about, and what just bugs you.