I think the main purpose of Azriel x Elain and the bonus chapter was to show us how they unknowingly helped each other and set up their future books.
Elain said she didn't want male, but quite clearly would have slept with Azriel in the bonus chapter. I'm think Azriel fucked his chances when he said "this was a mistake" rather than literally anything else, because Elain obviously took it as a rejection and closed the door on that relationship by returning the necklace. But now she is in fact open to a relationship with a male, which is a huge step for her. Lucien has a chance, but let's see if he's the first. My crack theory is Elthazar happening before Elucien. Balthazar is basically an Illyrian Lucien. He fights with his words and is against unnecessary violence, plus he saved Nesta so there's his in with Elain. Its a perfect spanner because if Azriel's getting his own book/mate, then there needs to be a new obstacle for Elucien. Also, imagine the chaos if SjM made this happen and sparked a new ship war before the last book! The carnage.
For Azriel, he needed to go after Elain to show that what he felt for Mor wasn't actually love. It was infatuation and he was so easily able to switch it to Elain. He's chasing a mating bond because not having one confirms he's unworthy, and what happened in his childhood was his fault. So much trauma. He needs to meet someone who understands his pain, who feels unworthy too, so they can help fill the broken pieces of each other. Its codependent but most relationships are in romance books! So then, how fun for the second part of the bonus chapter to put his mate right in front of him, but he's too traumatised to see it. Gwyn wasn't given such a fleshed out, tragic back story to be shoved aside. And the spark in Azriel's chest was unnecessary if they aren't in fact mates. In HOFAS, Azriel states he has no mate and no partner, which works fine for Gwynriel because friendship can developed off book, but romance (in a romance series) cannot. So, time skipping forward with a recap on how "he couldn't imagine he once questioned if they were friends" is enough back story to continue forward. We'll get a friendship to lovers/healing story with the obstacle being themselves!
Well, I'll stop now! If you made it this far, then happy Friday! Hope all your dreams come true 😆