In Brave, it is addressed that Vikings (along with Romans and other Northern invaders) were the adversaries that forced the four clans to band together and end their petty squabble and enter an era of peace under their new Kingdom. In the fandom, fans like to name the Hooligan tribe as one of the Viking tribes to be involved, creating a long and complex history between Berk and DunBroch.
Valka has been shown to be a misfit to her tribe (much like her son), favoring reason over violence, and would probably prefer peaceful negotiation over war with Scotland over "the Viking way". Queen Elinor is shown to possess critical thinking skills that mesh well with her position of authority, being able to to put her husband, the lords and all of the other warriors in her kingdom in their place with little effort. This would allow a calm, less-volatile relationship between one another from the beginning.
It was made clear that Elinor's marriage with Fergus was strictly for traditional purposes, and the idea of previous lovers in Elinor's life (be they male or female) is not impossible. Aside from this, their relationship would simply extend to friendship, especially when concerning their children.