I'm glad you have such a great vet
Oliver had grand mal seizures, but mild ones (that sounds like a contradiction, doesn't it?). Before changing his diet, his lasted up tor 5 minutes, with another 15 - 20 minutes post ictal where he was disoriented, didn't have much balance, etc, so he definitely had some long ones. Oliver had idiopathic epilepsy, from the time he was young, and a brain tumor wasn't really ever considered, so it may be a very different situation.
The Canine Epilepsy list is a great resource: http://www.canine-epilepsy.com/
They have a mailing list (you can also access their archives) as well as resources on the website. It's a wealth of information and support.
DogtorJ has a ton of information about GARD, which is the diet that worked so well for Oliver. There's a ton of information about it at http://dogtorj.com
The Canine Epilepsy Yahoo group is also another source of support: https://groups.yahoo.com/neo/groups/k9epileptics/info
is the worldwide list and there's also the K9 Epilepsy group: https://groups.yahoo.com/neo/groups/k9epilepsy/info
I did leave Oliver alone. I had to work. Initially I confined him to a "safe" area - no access to stairs he could fall down, things he could get caught behind/under, etc. I didn't medicate him for various reasons. One thing that seemed to help was to put something cool on his head/neck during or after the seizure. His head would get very warm to the touch, and cooling it off seemed to help him recover. Other than that, I just sat with him and kept him from getting injured. It seemed to help to talk to him, pet him. They say they're not aware when it's happening, but it seemed to help him to know I was there - he seemed less stressed by the seizures.
Sending good thought for Eddie.