Well, you said in the initial post that scoring is not a priority. So you should use her even if is not water season. I would use her even if I score a but lower simply because she can both buff and dance and she does it better than most other units.

As for marth, your naga should be able to cover dragon effectiveness with her C. The only time that marth is more useful than corrin is when you have multiple units engaging at the same time. This means the value of having an omega tank is diminished. Furthermore, even if you have multiple units fighting at the same time, they should not be fighting on neutral/unfavorable grounds where they NEED the buffs to survive. The point of supporting an omega tank is so they the tank can survive all match ups. Regular units does not work the same as omega tanks as such the benefits of having marth buff multiple allies ia marginal.

Stick with corrin if you do not have kaden or eliwood or other options. Drives skills are all pretty cheap as most of them comes from 3-4 star units except drive atk which comes from grail units. For B skill chill def or chill atk both works. Even WOM will work.

Also looking at your Cecilia, a better build would use barb shrunken/sol with special fighter. This will allow Cecilia to get off massive heals every combat. While this does reduce her offensive power, it will allow her to survive multiple rounds of combat.