The decrease in seizure activity is most likely in response to pexion and the valium (anti-epileptics). I would not stop meds unless under direction of the treating vet.
The reason the vet didn’t explore the option of food allergies causing your dog’s seizures is that based on my knowledge, that it would be extremely rare for this to be true.
The vet addressed the issues that would cause seizures in an elderly dog. “All the information and money” you gave him was so he could determine what testing to do to diagnose your dog and prescribe treatment.
It may be best to keep his diet simple, maybe even a little bland, as certain foods, preservatives and chemicals may be actual triggers.
PS: There is a lot of inaccurate information on the internet, and even if you find some helpful information, it is not specific to your dog.
