I know what you mean, Freya was a blunder in that department. However there's a difference: nobody expected Zenjiro to not take a concubine, but Aura is a different matter. Double standard, I know, but it was mentioned several times before it's a thing in there.
Nah, not a blunder at all. It was a calculated risk, and the one doing the calculating was Aura herself.
The thing is that the political ties and trade deals that having Zenjirou marry Freya will bring to her country are almost incalculable. And it all starts with the ships. Freya's people have shipwright technology that she wants for their merchants and navy. This is a serious matter for their country because it enables them to project their power and influence more effectively.
In return, Freya's people need more sources of lumber as it is critically important not only for building ships, but just for keeping her people alive - they need it as fuel for keeping warm during the long and harsh winters her country experiences. It will also allow her people to build more of those big ships that Aura is interested in.
And if they're gonna follow through on these trade deals? Then both countries are gonna need them. Both as merchants and as naval forces.
Normally, you don't see two powerful nations building an alliance. Usually, they're direct competitors for one reason or another - like how things were between the USA and the USSR for decades after WWII. But instead? The interests of these two nations don't overlap - instead, they
dovetail with each other.
Freya's people have the capability that Aura needs. And Aura's people have the raw materials Freya needs. It's a perfect scenario for building a strong alliance between two powerful nations. Having Freya marry Zenjirou helps solidify that alliance and makes it more difficult for one party to stab the other in the back.
The maraschino cherry on top is that Aura and Freya are of similar mindsets and get along well with each other. There was some initial friction over Zenjirou... And there still may be. Aura is genuinely in love with Zenjirou, and he with her. But Freya is smart enough to know that she's basically an interloper in this relationship, and is playing her cards with that in mind. She won't cause any undue hardship for them because this is an opportunity for both her and her people that she cannot pass up.
Aura, Freya, and Zenjirou all knew what the risks were. And one of those was that the Royal Families of the Twin Kingdoms would redouble their efforts to profit off of the situation, because now it would be known that Aura and Zenjirou are open to the idea of concubines or subordinate wives. And, unfortunately, the Royal Families of the Twin Kingdoms already have their foot in the door with the issue of Zenjirou's ancestry.
It may seem that the Twin Kingdoms has Aura and Zenjirou over the barrel, but you're forgetting one thing: The magical jewels the Twin Kingdoms use to massively hasten the construction of magical devices...
...jewels that are commonly known to us as "marbles".
If Aura's people figure out how to effectively mass produce these, then instead of having the Twin Kingdoms have an edge over them, they will instead have an edge over the Twin Kingdoms because Aura can directly control who they sell marbles to and in what quantities.
This won't entirely eliminate issues of meddling from the Twin Kingdoms, but it will equalize the political power between them.