If Neris had happened, where would Eris and Rhysand stand with each other? Assuming Beron is killed off, Feyre would probably still want some sort of relationship with her sister if only to make sure she is alright. Eris is now her brother in law so will they visit? Would he try to convince her to go back to her mate or would he just not risk war?
oh baby! my thoughts are that if neris happened as a canon divergence and not an alternate universe, the inner circle only let it get so far because they didn't think nesta would (or could) fall in love with eris. they allowed eris to sweep her off to autumn because they felt assured enough in the strength of her mating bond to cassian that it was a failsafe to ensure she would want to return to him (and thus be back under night court control) of her own volition. the intent was for her to serve in some sort of covert spy capacity and engender a sense of trust between the autumn court and the night court, then they would dangle something else in front of eris—someone powerful and pretty and docile—so that he would grow bored of nesta and discard her of his own volition. because the inner circle fundamentally misunderstands both eris and nesta.
but with her time away from the night court feeling like freedom, and with her social skillset put to such empowering use amongst the autumn court nobility, and with eris proving to be so much more complex than the cruel heir the night court paints him as, nesta finds herself certain that she has no interest in serving as rhysand's pawn—that she is happier with eris than she's been since turning fae—that her bond to cassian doesn't feel nearly as strong as this connection she and eris have forged with intention. and with him seeing nesta for all of her truths and appreciating each one, and with her so seamlessly navigating both the autumn court and his life, and with him and his family finally liberated from his father with her help, eris finds that he no longer just wants to use her for her power—that he has finally found someone he can trust to stand by his side—that he absolutely will not be letting her go, though he never had any intentions to. love consumes them.
this was certainly Not a part of the inner circle's plan. now feyre has lost her sister, cassian has lost his mate, and rhysand has lost a weapon—all because nesta had the audacity to make her own choices. i see it becoming incredibly contentious between all of these actors. i also see eris being incredibly smug about it. about being chosen. about taking from the inner circle. about immediately recognizing their game in using nesta as a pawn and coming out the victor. about seeing nesta true, truer than her own family, her own mate. about knowing that they can't do anything now that he's high lord, not even challenge him to a blood duel. after centuries of enduring their judgment and aspersions, he is finally vindicated.
so when the night court delegation comes to autumn under the pretense of ironing out the terms of the alliance between their courts, nesta has to incentivize eris to let her do most of the talking (yes naturally that gets sexual) and to trust her not to be swayed when [insert member of the inner circle] inevitably corners her alone to get her to come back (this is more a product of his deep-seated sense that no one could ever willingly choose him than a condemnation of her loyalty.) i think when the latter happens, he for sure has eyes and ears in that room somewhere, somehow. probably some charmed item from marius. eris is thoroughly warmed by nesta's conviction and loyalty to him, but he is furious about the way they're speaking to her—made even worse by the way that flame kindled to life inside of her begins to gutter over the course of the conversation. he can't keep his distance at that point, and the night court delegation is very promptly disinvited from their stay at the forest house.
it wouldn't end there by a long shot. nesta would want to see feyre and elain too so, at the very least, they would begrudgingly be allowed back. i think those visits are fraught and always occur with the undercurrent of all that isn't spoken between them, only for feyre to advocate for her return before she leaves because she misses nesta, and cassian misses nesta, and elain misses nesta, and if eris is keeping her here somehow they can help, and doesn't she want to come home? how could she be happy in a place like this? with a male like that? after enough of this, eris is pushed past the bounds of his civility and tells rhysand directly that if he continues to send his wife to try to take what belongs to him, the full might of autumn will be brought against the night court—insinuating, too, that his alliance with keir would see the hewn city rising up if he only requested it. that risk to rhysand's court would finally be enough to make him feign defeat, at least for as long as it takes him to devise some other way to get nesta back. for feyre, of course, and for cassian. not because he has ceded one of his greatest weapons to his enemy.