Merry Tanabemas @desfraisespartout! Here's a picture of Miguel and Robin getting birdmarried to fill your Robin x Bat wish!
Thoughts under cut
As to how the relationship is in this scenario... hm... I think Robin and Bat eventually get married post canon for political reasons. In my complex postcanon worldbuilding marriage isn't an institution that birdmen really participate in, and while you can have a ceremony, being married confers no additional rights to you. Most rights are either rights of an individual, or rights of a flock.
This is more of a show for the humans: marriage is something they understand. I don't think the stain on Miguel's reputation as "violent" will ever really go away, and he becomes the face of the negative stereotype of birdmen in the public consciousness.
Robin's faction obviously never even considered something like this, but as new birdmen and flocks began to pop up around the States, some began to spread rhetoric that birdmen as a whole should just abandon the violent and uncontrollable beasts like Miguel to the police entirely. When this thought persisted to a point that it was actively damaging efforts to unify birdmen in the US, Robin and Alva (it was mostly Alva tbh) made the decision to stage this wedding as a show of unity, that birdmen would not abandon their abused or mentally unstable members.
Robin, after all, is the face of the good birdman; she was the one in the photoshoots, who made the speech at in Washington DC, who was respectable in all ways. Their marriage is essentially Robin putting her stamp of approval on what Bat represents. Did it work? For the most part, yes, but there remains a small but conservative faction of birdmen who still kind of think those violent Latino birdmen are fucking it up for the rest of us even if they don't say it out loud. Sad but unfortunate.
To be totally honest Miguel had no say in this. Alva and Robin decided this on their own and kind of dropped it on him during one of their meetings and he couldn't really do anything about it (not that he really objected... it's not like he was in love with someone already, it's just. you know. shocking. I don't think he's someone who ever had marriage on his radar because his own parents' marriage ended badly)
But I do think Miguel and Robin have a special connection... Rooster of course also knew Arthur before he died but I think he looked up to Arthur more as a mentor. Miguel and Robin both treated Arthur as their leader so they still kind of had him on a pedestal, but I think both of them loved Arthur in the same way. This I think is the crux of what Robin and Miguel's relationship means to me. There's nobody else who understands my love. It isn't for you. But you're the only one who gets it.
I don't think they particularly WANT anything from each other in the relationship, either; they don't particularly want to be loved, or even feel that they deserve to be loved. They love each other as one might love a grave, because it was proof that the one who left had been there and it had all been real. Robin leverages the entirety of her political power to stand by Bat. He takes a bullet for her. They leave room for Jesus when they dance. And when they cross the dance floor you can see the Arthur-shaped negative space between them.
