You asked many questions.
Many are going to cruise on th eDawn this upcoming January. But I will not.
Yes, you can make all your reservations for dining at once. With that small a party, the main restaurants will probably just seat you as you arrive, but the speciality restaurants will take reservations as soon as you can get aboard.
Sea conditions can be good and bad all year. You'll have as likely a chance for smooth seas in January as you would in June. Storms cause rough seas, and there are storms all year.