I have to say that even though fresh greens always is the best option for healthy bunnies, I’ve found dried stuff to be a bit easier on the stomach. I don’t know why though, it’s just an observation I’ve made. I’ve dried just about anything driable and given to Bam. I’ve even dried kale.
Dandelion greens are traditionally used for gastric trouble in humans and so is mint. I don’t know if the BB store has hay with dried dandelion greens in it, but I know there are such products on the market. BB has hay-toppers I think, maybe some of that would be a nice supplement.
I agree with Tanlover, you should ask your vet about what you can feed her to up her weight. As for sunflower seeds they are used here as well for keeping good weight on bunnies housed outside during the winter, but they are extremely rich, full of fat and proteins (which is why they are used, of course) and I’m thinking they could cause her stomach to go berzerk again.