I wouldn't regard the schedules for April next year as being final, unless aircraft utilization suggests there's no scope for change.
In the meantime, I got to use the KEF service last week. As it was 3-4 weeks in, there shouldn't have been any issues with lack of return passengers on a flight. Both flights had 70-80 on them, which isn't great. One flight was operated by an A320, the other by an A321. That number of people really seems lost on an A321. Notably, the bulk of passengers were American rather than from Ireland or Iceland. If anyone is looking at booking travel, KEF is a much nicer airport to connect in than Heathrow or Schiphol.