Who to give DC?

F!Corrin is amazing with DC! Synergizes well with her Breath and she doesn't really need a special a slot to fulfill her role.
Beruka can't use it at all, since you have to give up your A and S slot just to deal with archers and that isn't worth it.
Kana is better of with Lightning Breath if you want him to be an all counter, PA!Azura and NY!Azura aren't making good use of it as well.
Ceada might be an option, however her niche is that she can deal with armors(almost all are melee) and horses(the most used ranged ones are Reinhard and B!Lyn, both of them kill Caeda with ease)

--> From the ones you said I would definitly go with F!Corrin, since she is easily mergeable and uses DC very well.

If it were me, I'd probably go for M!Kana. It would work really well with Water Breath and the other skills you mentioned.

As for the others, Caeda would probably prefer Swift Sparrow 2 over DC, Beruka can't tank mages as you said and since a lot of ranged attacks are magic.... (and bows unless you gave her Iote's Shield) it would only be good for daggers. I don't get the point of DC on dancers but your call. And F!Corrin... I'd probably just go for her son like I said.

Hope this helps!

F!Corrin, you can put Lightning breath on Kana to make him work well.

I vote for F!Corrin as well.
I think that M!Kana will be better off without it, you can use Lightening breath for that in combination with brazen skill.
If your armor emblem needs it, i guess you can use it there.

Caeda cant use it, she is great against magic and all, but you need some deffence as well to use DC wich is missing in case with Caeda.