Your cat's behavior reminds me of mine. I was feeding my cat the Senior Nutro dry food and he started that kind of vomiting that you described. I wonder if you considered his food? My cat is now eating of all things just a store brand Purina Naturals and is doing FAR better.

PS Just saw the update post, so it was the food! Wow, that's kind os interesting as that confirms what I expected with my cat. I'd also been giving him a turkey/giblets Fancy Feast that really made him vomit. A food allergy...hmm.