Hope I can help, @sunviking90 no need to pre pay gratuities, I think you need to call Celebrity. April 15th reservations should have been available a long time ago. If specialty or setting a preferred dining time is important to you please call asap otherwise this can be done once onboard but you may be limited with choices of time/days.
You can book the MDR for the full sailing as well. I have personally done this, book each day in the venue of choice, the same as you do for specialties. So for the future @The Med you now know.
Patty