What worries me the most about Molly and her symptoms is her age. I don't think it is so much what she is being fed that is the problem, although smaller more frequent meals are probably better tolerated. In older pets, there is a greater risk for her GI symptoms to be due to organ dysfunction, like kidney failure or even worse, things like GI cancer. Inflammatory bowel disease could be a consideration as well and is often a little easier managed with medications.
I would definitely recommend that you start with and exam and bloodwork at least. If the cause is not identified there, you can discuss the benefits or further testing or maybe a trial of medication. Pepcid AC can be helpful, but is really not much more than an antacid, so unless the problem has to do with excess gastric acid, it wouldn't be expected to help much.
In the meantime, continue to offer small amounts of water at a time and offer frequent, small meals of something very digestible for her. These would include items like boiled hamburger or chicken and rice or pasta. No sauces or gravies, but a little non greasy broth would be OK.
Good luck with her and please let me know if you have any further questions.