if you created a realtime incident then it is not possible to edit the start or end time later in edit mode.
This is not useful if you find out that an issue was already longer present since you created the incident.
btw: maybe it would be better if you not offer the switch between "realtime" incident and "backfill an old incident".
Could be confusing for the on-call duty process if you are under fire.
What about entering always start and - if incident is already solved - end time? If no end time entered then incident is still active.
Thanks and regards,