YES THIS IS CANON
I think about this all the time. He absolutely ignores Childe completely to hang out with his little siblings when they visit his family and Childe pouts because he usually shows up with all his expensive presents and crazy stories and gets all the attention but now his boyfriend and his siblings just hang out doing crafts and chatting (talking shit about him, according to Scara, but probably not) and he's stuck bringing them snacks and stuff lol
But also I think Scara would resist the idea of meeting Childe's family and be really standoffish at first, because he was really anxious about meeting them and finding out whether he'd actually fit into Childe's life - there's no way he'd stay with anyone who didn't get along with his family, and the stakes would feel so high he'd probably get avoidant to mask the anxiety. And then an hour into the first trip, he's sitting on the floor of Teucer's room meeting all his stuffed animals and they've never talked about this at all, and Childe has never really given much thought to his life more than a year in the future since everything that happened to him as a teen, but now he's seeing how Scara might be with their kids and suddenly the future feels really, really important.
(His mom convinces him not to propose on Christmas the first time he brought him home after officially dating for like six months but he makes sure he has a ring ready after that)

This is literally the sweetest thing omg
Honestly at first I struggled a lot with both Scara and Childe. They're very complex characters in that they're made up of these seemingly completely opposite/incompatible elements - being this sweet, loving guy but also a bloodthirsty weapon; or in Scara's case, someone who (in his opinion) is above every mortal being and would do just about anything to anyone to get what he wants, but somehow also (sometimes) is so deeply kind.
If we're being super honest, I identify way more with Scaramouche, so it's possible that's why his voice comes naturally to me haha

But on a more technical level, I think it helps to keep in mind that (oddly) the core of him is just...yearning. Not in a "simp" way, but in the sense that his whole identity is crafted around this feeling of emptiness and feeling incomplete. /Everything/ has to come from there. Is he being an asshole? Well in his mind, he's missing the part of him that wants to be nice (and the part that deserves to be liked/loved - so doesn't it make sense to hurt people and push them away?).

Similarly, when he does reach the point of actually wanting to connect with someone like he does with Childe, it's overwhelming because there's this realizing that that connection will fill in the void he's been trying to fill on his own. And that's not necessarily a good thing - because if your whole identity is based on being empty and superior and unlovable, what happens when you fill that space with love from another fragile, fallible person?

...I'm not sure how much any of that made sense, but it's what's in my head when I'm writing! I go back and adjust his internal monologue quite a bit to make sure it's balanced once I've got the shape of a scene down, as well. It's really mostly instinct at some point.

Ooh this is such a great question...I've been so focused on what he would look like that I haven't even begun to explore what he'd actually be like as a Harbinger.
I think it would take a lot of work, but he's probably just fall into the same role/rank as Arlecchino - she's grooming him so specifically, and she might even offer him up as a replacement if she wanted to retire imo.
I think he'd struggle a lot with it, as well. He'd take on the role of Father for the Hearth quite naturally; taking care of vulnerable kids would be second nature. But I think he'd have some serious reservations about the Tsaritsa and everything involved in being (essentially) one of her lieutenants. I just don't see him being able to convince himself to believe in The Cause (like Childe), or really lean into using the position for his own gain like many of the others. For him it would be a necessary evil to protect and fund the Hearth, and not much else.
Unfortunately I do believe he'd be incredible at the job. He's already got all the characteristics of a great diplomat - charisma, intelligence, the ability to read people. So I imagine he'd quickly become a favorite of the Tsaritsa and the main face/representative of the Harbingers.
As for relationships, I think Freminet and Lynette would have similar reservations. Lynette seems like she'd be more able to rationalize it all as a necessary evil to take care of the kids, though, so she'd probably be the one Lyney went to when he needed someone to reinforce that he was doing the best/only thing he could do. Ultimately it might strain things a little with Fremi, though.
Wriothesley...would get it, but absolutely hate it. He's the poster child for working within The System to fix it/make the best of it, but at the same time, being a Harbinger is on a completely different scale from being the Warden. Everything Lyney does has the potential to have a massive international impact and I think that would scare the hell out of him - especially when it could easily make Lyney a target for all sorts of plots and political intrigue.
I think the Wrioney situation would be affected by what stage their relationship was in when Lyney became a Harbinger ultimately. If it happened while they were in an established relationship, there would probably be discussion of how he might get out/Wriothesley would bring up his concerns openly and offer a counterpoint to Lynette's more pragmatic approach. But if they hadn't discussed their feelings...well I feel like I could write a horribly angsty fic about that tbh 👀 It would create such a conflict of interest and put even more distance between them...

Thank you for the prompt, thinking this through "out loud" was fun!