I am sorry to hear your dog is gone, and that you are so distressed.
I do not think the tick that you found last April caused the symptoms you describe. Seizures are not very common in tick borne diseases, but when they happen it is not 3 months later. I can't say for sure without scans like CT or MRI, but a brain tumor is probably the most common cause of seizures in older dogs. Phenobarb may help control seizures caused by a brain tumor for awhile, but eventually the seizures become worse, phenobarb does not help, and other neurologic problems such as head tilt, circling, etc.
Only a spinal tap could confirm if this was due to tick disease, and only a brain scan could have confirmed a brain tumor, but I suspect it was a brain tumor. The vast majority of tick bites do not result in any disease, and I think you would have seen something sooner than July if the tick was found in April.
Most tick diseases are spread by a tick that is attached for at least 24 hours, usually 48 or more, so having the tick removed as soon as you found it was the best thing to do.
I am sorry you feel so bad, but I suspect that this terrible illness and symptoms would have happened if the tick was never there.
Let me know if I can help with or answer anything else.