Before Answering about Daemon in the Fanfic
This post will have spoilers.
I also do not think canon him slept with Mysaria while married to Rhaenyra, ultimately because I feel and have faith that he was just too into her, had built a strong enough bond of trust for him to actually feel tempted, and because of Rhaenyra's mental condition at the time. I'm less confident about Mysaria all the same, while ainomica expresses total disdain for legitimate reasons HERE, some of them why I think he couldn't have slept with Mysaria.
Sweetestpopcorn's FanFic Daemon
However, in SP's fic, the Velaryon boys don't exist and Daemon and Rhaenyra marry much sooner. They don't actually build that mutual trust together yet, since Daemon sleeps with 2 sex workers gifted to him for the night by an Essosi merchant I forget the name of early in. So the possibility where Daemon cheats is more plausible with how early it was, who it was with, with them being married right after he "takes her virginity", etc.
In the fic's beginning (or really where we see what happened after he came back from the Stepstones in 111), we learn that Daemon seduced Rhaenyra more than he actually wanted to directly engage with her or saw her as a person. Basically that he only pursued her for power, nothing more.
However, later when he does fall for her in the fic (or acknowledges that he already did), we get a flashback to him holding baby Rhaenyra for the first time and SP makes it clear that there was always a soul connection between the two. And while he cheats with the two sex workers, he can't put Rhaenyra out of his head. So SP seems to want us to believe that Daemon represses his emotions and thought processes about them to himself in order to address the world around him as "realistically" as possible and protect himself from emotional damage (example, Viserys causing Mysaria's miscarriage).
And for the longest time, SP had Daemon continue to think to himself that he'd just rule Westeros through Rhaenyra and use her as a means to an end while indulging in the cookie, and then there's a point where he decides that being just being a Royal Consort and the father of the next King is enough.
Because if Daemon is more of a "realist" and a more ruthless person than most Daemyra shippers think -- one who doesn't express his/others' emotions or even think about them with a philosophical, "deep" analytical mind -- it's easier to present a Daemon who looks at the world through his feudal/male/princely privileges and having multiple sex partners while married to his first wife also cheating on the person he comes to realize (thought that cheating) he cares for much more than he ever thought he could love a person. That, and how the V boys don't exist, so SP can then make a story where it becomes easier to imagine canon Daemon actually having made plans/executed plans to kill Rhaenyra's kids by another man to make way for his own.
While reading, it felt wrong and right simultaneously. Mostly because I don't see Daemon fooling himself that hard -- that Rhaenyra is a device -- or for so long. Or that he'd actually be the person to actually kill his wife's kids (if they had existed) for his own son's ascension. But I can see that there could have been a progression towards a realization of the depth of his feelings for her, that there could have been a progression at all I mean. That there was plausibility in that aspect.
The Daemon I got from canon is ambitious and I can see him as a guy who doesn't think or ponder over deeper emotions and motivations, including his own. Again, very action-oriented guy. Doesn't analyze his own emotions, just lets them happen. He sees that multiple people in arranged marriages have lovers, he had a terrible marriage with a woman he did not like much less love and is as "restless" as HotD Viserys paints him because he wishes to escape those bonds. Marriage = containment/becoming a means to an end for others.
So it's easier to buy into SP's fic Daemon and conclude that Daemon felt that his marriage with Rhaenyra was an opportunity to escape that marriage/a life of emotional imprisonment. When you focus on them focusing on Daemon being distant from Rhaenyra as he's portrayed to be. By the time he gets to Rhaenyra, he has been used to the idea that marriage is only good for politics and power, has used these sex workers for pleasure, destressing, and he is a feudal man in a world where men have the allowance of doing all of what I said over women. However before marrying Rhaenyra, Daemon also sought out Mysaria and kept her around in both in canon AND sweetestpopcorn's fic because he connected with her and wanted intimacy, something close enough to love. Still, the Daemon of SP's fic is a Daemon that has reduced or objectified his own sexual and emotional experiences into materials to handle according to his own will. Him marrying Rhaenyra in the fic is mainly him taking back control and making marriage -- the feudal institution itself -- less of an imprisonment and more of an advantage for him. As marriage is treated those who arrange them in the first place.
And to hear SP's Rhaenyra explicitly demand he not sleep with anyone else with the threat of sleeping with others, I saw that SP was telling their readers that Daemon sleeping with the two sex workers was his act of defiance turned into the beginning of an epiphany, or a loosening of his self awareness.
Actually Answering the Question
While I see the vision, SP already warns their readers that they deviate from canon at a certain point, and I see how it does use canon traits for it's characterization, I don't really agree with it.
Personally, I didn't like that he cheated and screamed at my computer when it happened, since SP's Daemon was much more self-centered than I imagined (and I already felt that Daemon was his own selfish and sexist person, since he does describe Rhea Royce in demeaning, gender-based ways AND there is an element of his character where he was pursuing Rhaenyra for power -- both consequences of being a dragonriding feudal prince/man).
I also saw how SP wrote the scene to exhibit both his messiness and how he uses action-before-thought (more often) to come to a realization (for personal things) that he doesn't even fully acknowledge so much as falls into and accepts because he doesnt want to invest his mental energy into it when he feels so strongly already.
Ultimately, I'm not totally down for this storyline despite its plausible elements because Daemon is more of a douche to Rhaenyra than I felt he was when I never really thought he would. One could say that his time away from her in the Stepstones and his running from his emotions might make more room for him to be so douche and self-serving, more dismissive of her as a person in her own right after having thought more of what he needed after he baished Viserys in his heart after the miscarriage (that's what happens in the fic, he doesn't really see himnas a brother anymore).
But when it's couched with his repeated dismissals of her other protests against his thoughts or behaviors AND her obvious belief that he is her one and only, that he'd never go out of his way to hurt her or do something he'd know would hurt her WHILE us, the readers, know that Daemon cheated on her....I am and have grown suspicious of the perspective.
The war has begun in the fic, so I feel that SP is building towards a huge, potentially bond-breaking rift between R and D with Rhaenyra to match their disintegration in canon that I dread, especially since they utterly removed Jace, Luke, and Joff, even adding a child between the that doesn't exist in canon.
You know, apart from my love for Rhaenyra and me wanting only the best for her in all things without removing all conflict (internal and external) entirely.
Better than HotD still since the fic actually follow the world building pretty well where it needs to, and the writers sticks to their guns, staying consistent.
