Your vet sounds so helpful
I'm sorry you're going through this. Reactions to any food can kill. I have never heard of that from canned pumpkin. Have you tried plain yogurt (something with lots of active cultures like Stonyfield) and banana? I don't know what to tell you except that you might want to find a new vet if this one acts like this often. Vets may not always have the answers but they could atleast explain why they say the things they say.