I am so sorry for your loss also. It is heart wrenching to lose them, no matter how it happens. Her behavior changes do sound to me like she may have had a brain tumor; my Siberian husky died of a brain tumor. Though hard for you to see the aftermath of, during a seizure, the animal is out of it. It sounds like your beloved dog did not regain consciousness, so I think you should be encouraged that she was not aware and in pain.
Innoculations can cause reactions, but I think a brain tumor was the primary cause, and all you did was follow the law and protect your pet from rabies, as we all must do. You have no reason to feel guilty, although I understand your brain is searching for ways you could have prevented her death. It's what we do.
Holly and you shared a wonderful bond, and you were the love of her life. She was lucky and happy to have shared it with you, I am very certain.
I have no idea, bu I wanted to say I'm so sorry :( I know how hard it is to loose a loved pet so unexpectedly. (((((((( hugs )))))))))