When are scheduled reminders for late registrants to events sent or not sent?
Good question with several permutations! Here's a partial answer from experience.

Case A: A contact registers after an event scheduled reminder was
sent, but before the event starts.

It is sent. The timing will be whenever your Send Scheduled Reminders scheduled job runs. It occurs when you set x time before and based on this answer specific time it will not.

Case B: A contact registers after an event scheduled reminder was
sent, but while the event is happening.

It is sent.

Case C: A contact is registered for the event after the event ends.

Not sent.
I've not seen a 24 hour test.