Yep, I remember you using the digestive enzymes and the results. I'm wondering (and this is all speculation based on what I'm currently learning) if the reason they weren't as effective is because (again, speculating) she has low stomach acid. Without the acid to break down the food, the enzymes have a hard time doing their job.
L-methionine is an amino acid. This is what I got from WebMD:
Methionine is an amino acid. Amino acids are the building blocks that our bodies use to make proteins. Methionine is found in meat, fish, and dairy products, and it plays an important role in many cell functions.
Methionine is used to prevent liver damage in acetaminophen (Tylenol) poisoning. It is also used for increasing the acidity of urine, treating liver disorders, and improving wound healing. Other uses include treating depression, alcoholism, allergies, asthma, copper poisoning, radiation side effects, schizophrenia, drug withdrawal, and Parkinson's disease.
So it would acidify her urine, but not necessarily the tummy.
Going back to the betaine, the more I thought about it last night, the more I wasn't sure about it, because I don't know if it's safe for cats. I don't know if anyone's even tried giving it to a cat. From what I understand, though, it's synthetic, not a natural supplement, which is why I'm not sure about using it for Mazy.
So I would lean more towards the ACV (with the mother in it -- ShopRite sells the Bragg brand which doesn't have the mother), as that's definitely natural. If I wanted to try using it, I would think maybe take a dropper and add 1-2 drops (I forget what her meal sizes are, but I think they're small) and mix in her food and see how it goes (assuming she'll even eat it). If she eats it and nothing happens, increase by a drop, and so on. But this is all me just guessing here, and I have no desire to inadvertently make things worse. I've heard of people adding ACV to their dog's food (I don't remember why), but cats are not dogs.
Side note: one of my posts mentioned that we produce less stomach acid as we age. I have NO idea if the same is true for cats and dogs. I tend to think that, when it comes to things like this, what's true for one mammal is true for another, but that's just me.