Firstly im extremely sorry to hear that and hope very much the seizures will not continue...
One thing that I do not understand however is a vet puting any dog on medication after only one seizure

The reason I say that is that I thought it was the 'norm' to monitor the situation first of all and only if seizures are happening more frequently (ie 4-6 weeks apart) do vets then decide to medicate.
How long after the seizure did Monty have his tests - did he first have blood tests before the different scans? Often seizures can be caused by a number of different things - even over heating & dehydration can be a cause.
My gsd had a seizure (grand mal) last May and another 5.5 months later - 8months since the second so far and going well - but is he not on any medication as the vet was happy with the gap's inbetween - obviously if they became more frequent then that would change. The medication I believe (if it is phonbarbatol) can have effects in itself (not to worry you saying this) such as long term liver damage - hence why vets keep a check on the seizure pattern first - they can decide the best form of treatment, amount of medication etc in order to control the seizures and to best suit the dog.
I personally would want to know abit more about why monty has been medicated so soon with no monitoring. Idopathic epilepsy is a term given for epilepsy when there is no known cause found is more then likely a gentic problem in a dogs lines. A single seizuring dog and an epileptic dog are two very different things in my humble opinion.
I wish Monty all the best...and hope he stays seizure free xx