Hi! What syringe are you using to feed her? It has to be a big one, with a wide nozzle, or the stuff will get stuck. One like this:
http://www.towervets.co.uk/pet-factsheets/images/24_290699.jpg
If the syringe is the right one, then you can try to mix the hay powder with lukewarm water and mix very throughly, so that you will have a softer paste. As you fill the syringe, push the air out little by little and tap it on the table to help the paste move around and have the air out.
When you feed, push the syringe deep towards the cheek opposite as the side you are entering from, that way the paste will be in her mouth and it’s less likely that she will spit it. If you do this, you have to push very little amounts every time to give her the time to swallow it. Try also to tempt her with some smushed banana to see if she’s willing to eat by herself, that would be a very positive thing ^^
My bun had troubles with his teeth as well as one big abscess removed from his belly, so I understand how hard and stressing it is… keep track of how much you manage to feed her and how much she drinks, in case she will later need subcutaneous fluids admininstered. Good luck!
About the thing in her belly, the vet didn’t take it out yet, right? Does they know for sure it’s a tumor and not a really hard abscess? If they haven’t done a biopsy yet, I would ask for that so that you have confirmation of what the mass is… then I would discuss the chance to have a surgery to remove it.