Spoilers
Actually no. You're correct on the fact that Blake and Diana won't like each other in a romantic way, just friends. But, after Ancia realized that and her feelings for Blake (idk when was that bc there was a time skip) she decided to just keep them as friends and study on how to lift the curse.
She won't leave blake?
I think so, she acknowledged her feelings for him and she doesn't want to leave him, nor want him leave her, hence she's finding ways on how to remove his curse. I haven't finished the novel yet so, I don't know much.
I feel like blake won't like diana in a romantic way but as a brother-in-law and ancia will still ship or step-up them thinking that's the one and only way to remove the curse from blake (or she may think that blake is destined to like diana just like what happend in ze novel) .-. Dang it...