I don't have any advice for you except to post your question again in the food forum (at the top fo the page, click on "Discussion Groups - mainly cats and dogs", scroll down to "Dog and Cat Food Forum" and click on that. Start a new thread with the "New Thread" button you'll see at the top left of the listed threads.) A lot more members will see your question there and you'll get more responses.
I do know how scary grand mal seizures can be
and I hope between the phenobarb and any diet changes, you can get them under control quickly!
Welcome to the board.