But there's also the question of when they take place during the game. The support between Seth and Eirika has always seemed to me something that took place in the early chapters of the game. This mainly due to some lines in the A support. There's Seth's line that "This thought should not rest beneath the crown you soon must wear." And then Eirika comments: "But... it was improper for a queen of Renais. And with my father dead, I suppose that is my station. I am a queen, and you are a knight in my service. If I do not keep this in mind, then I will never be able to rebuild our kingdom."
I admit, the kingship/queenship of Renais seems sort of wonky sometimes and maybe I should do a post on that too. Basically, though, I see Ephraim as the older twin and thus the king, leaving Eirika as a princess... unless something has happened to him. So in my mind, if Seth and Eirika are both thinking of her as the queen then it's more likely that the conversation takes place when Ephraim is still missing behind Grado lines.
Similarly, I always had it in my head that Tana's support with Ephraim takes place earlier in the game. She seems much more childish in those supports versus her supports with Syrene which have to take place later in the game a Syrene only arrives in chapter sixteen. Some of her other supports are like that so I mentally arrange them in a certain order.