I have to say that your number one and number two are just that.
A person having a seizure has a far less chance, almost zero, of injuring another passenger than a mobility impaired person running their "little rascal" around on deck or maneuvering it in close quarters. I've been injured by those more than once and I wasn't on the ground having a "fit" at the time.
I've never seen a mobility impaired passenger removed from the ship because they could be a potential hazard.
Also, a passenger is not going be allowed near ANYTHING that would disable or disrupt the ship's systems.
Number three is possible but highly unlikely. A person having a tonic-clonic seizure is not going to "jump" or hurl themselves over the railing and they aren't going to fall over the railing unless they were leaning over it at the time of the seizure. They will lose motor control of their legs and collapse. That's like saying people "fall" off a cruise ship. I don't think anyone has actually fallen off a cruise ship unless they intentionally jumped or were making whoopie on the balcony railing or doing something else quite stupid and quite conscious.
These are are the fallacies and fears that have kept people with Epilepsy in the dark for centuries.
There are so many types of seizures and many are "relatively" benign but tonic-clonic seizures, in clusters, can be very dangerous to the individual. They are the most dangerous, to the individual, because of the possible loss of oxygen and the injury they could inflict upon themselves during the seizure. Head injuries are the most common. Also, no one with a seizure disorder should operate a vehicle or machinery unless they have their seizures under control and have been seizure free for an amount of time deemed safe by their doctor and the laws of their state.
In my previous post, I said that if there were information that the ship's doctor and Captain had that we don't (i.e. he had three seizures within twenty minutes) then it could have been a reason to have the young man leave the ship.
I base my opinions, in this thread, on the information that he had only one, tonic-clonic, seizure and he had not taken his medication that morning.