I agree, it is terrifying to see a seizure. Complete thyroid panel is a good start, and keep a careful diary, starting today, of the time of day, length of seizure (it only seems like forever) and recovery time. Think back to anything new in her life (new carpet, detergents, lawn care products, etc). Then just wait and hope another does not happen. If it does, keep up the diary. Dogs may have occasional random seizures, most vets don't get excited till they come often, or are very prolonged.
A seizure uses a lot of calories, so the ice cream sounds like a good idea! Our old dog took about an hour to recover and she was disoriented but frantic to walk. We would leash her and go out into the street in front of our house and just let her walk in circles till she was exhausted, then she would sleep for a few hours and be just fine. She did that about once a month after she was 17.
Your dogs are both beautiful! I hope your lovely girl never has another seizure!