Mysterious illness and behavior issues

Hello, I wanted to see if anyone has some advice on what we can do moving forward regarding my dog with a mysterious illness and behavior issues.

We got butters (chihuahua) about 7-8 years ago as a rescue, she has always suffered from anxiety and was placed on Prozac about a year ago for it. She had seizures before but they increased dramatically when she was on Prozac so they cut it and she started phenobarbital. Her seizures have decreased dramatically and her anxiety has been manageable. But a month they cut Prozac Butters lost like 30% of her body weight. She was around 10 pounds but weighed in around 8.3 pounds that visit.

The theory was that maybe her anxiety or her seizures was increasing her metabolism. She still had a regular diet, thirst, and activity level. Despite phenobarbital and the Prozac, she is still very skinny. We have switched her to a high calorie diet to see if that’ll help. Another symptom was increased thirst. I’m telling you, she would drink unlimited water if we allowed it. If I put out a normal bowl she’ll drink it all and vomit it immediately. Since she’s so skinny she’s walking around with a bulbous belly from all the water. So we limit and take it away at intervals because she’ll literally inhale the water. I also have a second dog and this makes watering her difficult as there’s never water for her.

We had her tested for diabetes, kidney, liver, hay fever, Cushing’s, tick disorders, and might be a few more 2 times over the past 1-2 years. All came back negative or normal.

On top of all of this, butters will have bouts where she is completely un housebroken. She has to go every hour and you have maybe 1 minute to take her out when she wants to go before she goes in the house. It’s not even on the floor either, she’ll pee on the couch, she has literally shit on our dinner table TWICE. We go through SO MUCH paper towels with this. We have to crate her if we have to leave her alone in a room for a period of time. It almost feels like she purposely waits until we are not looking. Sometimes I will be sitting by the door and she’ll go somewhere to hide and go, rather than ask. She struggles to hold it at night as well. Either waking me up multiple times or going in her crate.

The vet’s next suggestion is an MRI to see if it’s neurological. I’ll be honest, we cannot afford that. We have spent thousands on her treatment so far and have left feeling frustrated. Her behaviors are out of control and causing problems in the house. I rent and will likely have to pay for new tile and new carpets because of her going in the house multiple times every single day. Because of the financial issues and my inability to manage these behaviors, I considered rehoming her. But due to her medical issues and her being a chihuahua I fear she won’t ever find a home. I wonder if seeing a different vet could be a solution. They never tested for worms, would that be readily noticeable?

I don’t know what to do. Has anyone seen this before? Any suggestions?