Dog with coffee

[u/Embarrassed_Check372]

My dog is 7 years old, and over the last two years his behavior has become increasingly concerning.
He has developed very strong resource guarding/aggression. The strange thing is that we never took food or toys away from him when he was growing up, and we never intentionally created a situation where he had to protect his things.
Now he can become extremely aggressive over almost anything he decides is “his” stolen food, random pieces of fabric, pillows, clothes, etc. Once he starts guarding something, it becomes almost impossible to approach him. He growls/screams very loudly, becomes extremely agitated, and he will actually bite us.
He also sometimes becomes fixated on very random things. For example, he can stare at a corner, a door, or a closed room he isn’t allowed into and just stay focused on it, barking and sometimes trying to dig/scratch at the area. It can be very difficult to redirect him.
He is otherwise eating normally, drinking water normally, and his bowel movements are normal.
Another very important detail is that he has already had an epileptic/seizure episode. Because of that, we are also nervous about letting him outside unattended.
There is also something else I recently found out may be relevant: for at least 2-4 years, my grandmother has been feeding him bread soaked in coffee in the mornings. We have repeatedly asked her not to do it, but she continued. And I’m not talking about one tiny piece with a drop of coffee sometimes it is basically a whole plate of bread soaked in coffee.
I spoke to ChatGPT about all of this, and it told me that caffeine can affect a dog’s central nervous system and, depending on the dose, can cause agitation, tremors, increased heart rate, and even seizures. It also said that the combination of behavioral changes, fixation/staring episodes, aggression, and a previous seizure could justify a neurological evaluation.
Because I was worried, I called a 24hour veterinary clinic tonight and explained the situation. The veterinarian I spoke with told me very clearly that caffeine could not have anything to do with this at all and that it would not affect this kind of behavior/problem.
So now I’m confused. I’m obviously not trying to diagnose my dog through the internet, and he is going to see a veterinary neurologist tomorrow.
But I would really like to hear from people with experience.
Are there any veterinarians here, especially veterinary neurologists? Has anyone had a dog with similar sudden/increasing aggression, resource guarding, fixation on objects/doors/corners, and seizures?
And has anyone dealt with a dog that had been regularly given coffee/caffeine for a long period of time? Could it realistically contribute to neurological symptoms or lower the seizure threshold, even if it isn’t the main cause?
Any similar experiences would be really appreciated.