I'm sorry it's so worrying for you, hip chick. Her crop sounds OK, as if food is passing through, which is good. It sounds like some kind of intestinal blockage, maybe egg bound, possibly an infection, or peritonitis though this is unusual in a young bird. Its good that she can drink for herself, and so long as she is seen doing this, there will be no need to syringe her, as this can be dangerous.
I would venture to disagree with the breeder about their worm count. All chickens have worms to some extent, although regular worming with flubenvet keeps them at a low enough level not to interfere with their health. Unless he had done a worm sample test on each and every bird immediately prior to sale, he has no grounds for saying they don't have worms. I routinely worm any new point of lay pullets I buy, during their two-week quarantine period, so they are not bringing in worms from a different environment. I would advise you to worm your other pullets with Flubenvet, and mention the possibility of worms to the vet as it sounds as if she's never been done. I don't suppose she has a massive worm burden, but any worms will pull a chicken down, reduce their ability to digest food and lay well, affect their immune system and may complicate other health problems.
This section about intestinal problems on Poultrykeeper may help; https://poultrykeeper.com/digestive-system-problems/
It's often very difficult to diagnose internal problems in chickens, as their intestines are so different from ours, so as you vet isn't an expert, he or she may probably resort to a broad-spectrum antibiotic such as Baytril. This may or may not help, and would mean that you could never safely eat her eggs again afterwards because of the risk to your own health of developing antibiotic resistance. It's a hard call, when an animal is a pet, but please read the thread at the top of the Chickens Health section, 'Is your antibiotic really necessary?' so you are informed in your choices. You'll see that quite a few of us have decided that antibiotics are not only ineffective for many complaints in chickens, but in themselves to be avoided for the wider good. If she seems on a downward spiral, it's often kindest to provide the hen with peace and quiet, checking regularly, and giving her body the chance to heal itself if this is possible. Quite often they do perk up when given a quiet rest. If after a day or two she seems to be in pain and isn't getting better, then a peaceful way out is to have her PTS by the vet if you aren't able to do this yourself.
Good luck, and do let us know how she gets on.