giving Diana a consolation love interest
Frankly, as someone who was immediately happy when I saw Alexandra being presented as a genuine love interest, over Diana continuing to pine after Natori... I'd say she turned out to be much more than that.
She's not just there to console Diana, or be consoled by her. Her role as a noble forced to suppress her talents and obey authority, being a pawn in larger schemes, while also joining a group of commoners, gives her one of the most complex character positions in the series, possibly second only to Lapis.
She also creates a bridge between nobles and commoners that the original Fairy Tale of Jewels didn't have, since Diana was the only one in that position. Alexandra "siding" with her own team at the end matters a lot - I can easily picture that, if Lapis' plan succeeds and a rebellion occurs, Alexandra, and possibly some other lower ranked nobles, might side with the commoners, shifting the balance (and story) quite a bit.
Oh, and yes - I'm definitely rooting for Diana x Allie. But Alexandra has also grown into my favorite character, and I hope she continues to be more than just "the love interest". Based on this arc, it feels like she absolutely will be.
Also, her character design is just perfect. She's not there just to be a background character.