I have had extensive (unfortunately) experience with PLE. Usually, in yorkies, it is lymphangectasia and/or IBD . Here is a link about my Cookie. She lived for about 6 years after being diagnosed and died from lymphoma, so it IS possible to maintain them.
https://www.yorkietalk.com/forums/si...ia-update.html
My Kiche has lymphangectasia and has been doing pretty well. He is on prednisone daily and a low fat diet. It has been a couple of years I believe, I would have to look it up.
Anyway, they definitely need low fat diet with diet being key in the treatment. They also are put on predisone and sometimes need other meds, one being a type of chemo.
I home cooked for Cookie because she had IBD and needed a novel protein so the Low Fat foods may not have worked, but Kiche is on Royal Canin Gastrointestinal Low Fat and he loves it. He is also on daily prednisone. They both were scoped which is the ideal situation but if not affordable, your vet should be able to manage her anyway.
You seriously need to get her back on the prednisone that the vet put her on!!! She may need additional medications to turn her around. Sometimes, when they are really out of kilter and have fluid in their abdomen, a hetastarch infusion may help. (its like plasma)
Her protein level is really, really low and I encourage you to get her back on her meds and on a super low fat diet. If you can do so, I highly recommend a consult with an internal med specialist. If you cannot do that, just make sure your vet is up to date in treatments for PLE.
I hope you can turn her around...she is young and could have a long life ahead of her like my Cookie did. I know other people with PLE pups who have also been treated successfully.