Fallen Angel!MC dislikes using their wings unless they absolutely have to. They aren't exactly fond of Diavolo's plan to unite the realms because honestly they wish both Devildom and Celestial Realm would stay the hell out of the human world. Humans already have enough problems to deal with on their own.
Though Fallen!MC is not related to Lilith at all I could see Lilith bothering them in their dreams like a creepy stalker ghost. Trying to convince the angel that they should really help save her family because 'it's the right thing to do' and then being told no. They're an angel of patience not an angel of kindness.
MC doesn't even really want to make pacts with the demons - they abhor the idea of controlling others. It would make them as bad as the Celestial Realm they fled. They do end up helping Belphie get free from the attic if only because they value freedom so much. Perhaps they also have a soft spot for someone who they once called friend. Though him claiming to be a human and fallen!mc pointing out that his portrait is in the hall.
Of course once Lesson 16 happens and everyone realizes that mc's a fallen angel because humans don't bleed golden blood. Nor do they smite demons with divine light. Or have broken halos that are blackened with tarnish. I imagine the brothers have varying levels of horror and disbelief at what mc actually is and what it means for them.
Diavolo and Barbatos trying to figure out what to do with this new information. How would the Celestial Realm and Michael react to the news that the missing angel has been found?
I doubt the brothers are going to want to surrender fallen!mc - they already lost Lilith. While they don't know the details surrounding mc's fall how do they know that they won't be executed if handed over to Michael. Most likely they'd try to convince MC to stay in devildom - which they won't. The human realm is there home - it's where they belong.
There's so much drama potential but also Belphie's self-loathing and jealousy to try and keep his fallen love interest in Devildom with him. Also demons trying to court the fallen angel and absolutely failing at it. They don't seem to figure out what would win the Fallen's affection would be returning them to the human realm.
Yes to all the angst!!! and also!!!! Consider the arc of a fallen MC who learns the pacts aren't about control: they're about trust. They're a promise, loyalty for loyalty. For a solitary creature, it's like a siren song.
Fallen MC distrusting Diavolo's desires for peace - because won't it just be more of the same? Except instead of a stalemate and a silent treatment between the realms, its just the tacit acknowledgement that both their systems suck and they're not going to do anything about it. And once they find out about Diavolo 'binding' Lucifer in his most vulnerable moments? ooh boy. oh no.
The brothers believing that 'hey, we're both technically fallen!' means they'll have an easier time courting MC, but in reality it's harder, because for all intents and purposes, they fell with grace. They had a new home waiting for them. They had each other - they weren't alone.