I personally think doing the endoscopy is the ideal way to go. You then would have more details. Often a pup with lymphangectasia will also have IBD.
In my Cookie's case, she did have both and I consulted with a vet nutritionist and I home cook for her. She has been on the same diet since she was diagnosed in Dec. 2011 and is doing very well. She was only taken off of prednisone this past year. They tapered it as we went along and her protein levels improved...and then they kept her on a low dose every other day until they finally took her off of it.
Sometimes they can have a fungal infection, histoplasmosis, so as I said, it is good to have the biopsies done so that you know. Here is a great link about it:
Intestinal Lymphangiectasia
Here is a link to Cookie's story:
http://www.yorkietalk.com/forums/sic...ia-update.html
Please keep us posted on Abby's progress. I will keep her and you in my thoughts and prayers.