I don't think it is an either or situation about love trumping the mating bond/free will/choice or Elriel will be mates. I think it is more of an all of the above type situation and we will have a nature picked wrong mating bond, rejection plotline, love trumping a mating bond, free will and choice, and 'true' mates.
So obviously, the elucien bond is nature picked wrong and I think Elain will choose to be with Azriel of her own free will (love will trump the mating bond) and reject the bond with Lucien. So after already choosing Azriel and love over a mating bond that is when it will be revealed that Elain and Azriel are 'true' mates.
I think the foreshadowing to all those aspects is purposeful and a sign we will get all of them. Plus, three sisters and only two get 'true mates' and the other gets a stinky unwanted mating bond seems unbalanced to me.