To me, your approach makes terrific sense. Your instinct that the seizures may be secondary to something else, perhaps TBD or infection/inflammation sounds spot on.
One thing my vet mentioned on the issue of doing an MRI to check for brain tumor was that, if a tumor was present, we'd know soon enough because of increasing neurological symptoms--grim but true. Seizure have an almost endless array of possible causes and when they are secondary to something else, treating that something else is the ticket.
Sending prayers and best wishes for Penny.
--Lucy