Sendings lots and lots of prayers for beautiful Coaster and for you.
I hate, more than I can say, to say this but you might ask your vet if this could be inherited. I say this only because I lost my beautiful Morgaine 2 years ago this week to a status seizure. He explained that it could have been an aneurysm or a brain tumor but I didn't do an autopsy. I did look at her background and her grand-sire HB's Commander died of an aneurysm. I don't honestly know if that is something that is passed down but it might be good for your vet to be aware of.
Please know that I'm not trying to scare you, I just think that the more informed you are, the better the vet can help you.