I'm glad to hear the seizures are not violent, but I am a little confused: she had a cluster seizure but we're not sure that was a seizure? Generally a cluster seizure is a series of seizures occuring close together so it should be obvious what is going on.
Roxy's dose of Zonisamide is right in the middle of the dosage range so not on the high end. If this drug is causing her sedation, then she may adapt over time. It isn't very common to have marked sedation on this drug but it can happen. If it persists, then you may inquire about lowering the dose to 50mg every 12 hours instead but please don't do that on your own. The valium is nice to have on hand just in case you need it for an obvious proglonged seizure event.
Although an MRI or CT would be required to fully evaluate the inner ear, a problem there would not cause episodes like Roxy is having.
An event monitor is ar portable electrocardiographic recording device that records a 10-15 minute loop of ECG, continually over-writing previously acquired data. Unlike the traditional 24 hour Holter, an event monitor only records ECG information when the unit is triggered (manually because you observe an episode and press a button on the monitor), the ECG from the 5 minutes prior to the trigger, and 5 minutes after the trigger, is transferred from the buffer into memory, and can be subsequently downloaded and analyzed. If episodes are infrequent (i.e. not daily), then the event monitor is the best way to assess them. Roxy would wear the event monitor as long as needed to capture an episode: it is a little smaller than the Holter monitor. Please note that the 24 hour Holter monitoring is also helpful and necessary for Roxy to assess how well her arrhythmia is controlled. However, to get information about if an episode of collapse is cardiac or seizure related, the event monitor would be necessary.