I've dealt with 2 rejecting mares. Both times a little Banamine and Ace helped a LOT. With the one mare, she was just being an idiot and was a maiden mare. I gave her the shots and then after about 20 mins brought the foal back to nurse and she tried to kick it away again, I got tough with her and let her know that while that foal MIGHT eat her alive, I would most SURELY do it if the foal didn't get to nurse. Took about twice and she was fine.
The 2nd mare was tougher. She didn't produce much milk and the baby's demands made her sore. She rejected 2 foals prior to mine, for another owner, and when she tried to reject mine, I did the Banamine & Ace thing and she stiil wouldn't allow baby to nurse. We ended up having to supplement with buckets of Foal Lac though to make up for the lack in milk.
The 2nd mare, we made an "Amish Stock" out of hay bales. Basically, we just walled her in with the hay bales and left a small opening for the foal to reach through and be able to reach her teats. We tied up the opposite rear leg and eventually, we hobbled her so that if she kicked she pulled her front foot out from underneath her too. After she upset her balance a few times she quit kicking. We also nose twitched her for the process and eventually, we could bring the foal up for about 10 mins at a time, once an hour, and let the foal drink.
For the 2nd mare, it really wasn't her fault, she just didn't produce a lot of milk, even when we used Domperidone to help her. I still have her, she is a good mom, as long as I supplement her milk with Foal Lac, she now accepts her babies right away and lets them nurse. I just have to make sure the baby doesn't demand so much and so hard that she gets sore.
The first mare, tried to reject a 2nd time, got the snot whipped out of her and she became an excellent mom. Finally got through to her that rejecting wasn't even a possibility. Kind of sounds like what's going on with your mare. You're managing the foal end of the problem real well, but I'd be having a "come to the waters" talk with the mare. If she didn't come back singing, "Glory Hallelujah", I'd sell her, with full disclosure.
This might take some of the work out of getting the foal fed: Miniature Donkey - Short ASSets Ranch - Miniature Donkeys - My Momma's an Igloo