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!

Oh I have a whole post about this a while back but in my mind each ship has like...a default position that sort of encapsulates their vibe? If that makes sense? It almost represents their dynamic
Anyway for Wrioney (hear me out!) I think that position is Missionary. They're so disgustingly in love that I think they could get off just from making eye contact tbh, and Lyney would feel especially safe with Wriothesley over him like that. It also allows them to kiss the whole time and you KNOW they're a "let's just make out for a couple minutes...oops that went further than I expected" kinda couple lmao
I could also see Lyney sitting in Wrio's lap being a favorite, because it's still very close & intimate & good for kissing, & Wriothesley would enjoy the view haha. This would probably be one Lyney enjoyed when he wanted a little more control, too - although in my mind something he loves about sleeping with Wriothesley is that he doesn't feel like he needs to be in control because he /is/ so safe in his hands.

cackles evilly yes...YES...

OK SO I'm actually working on a fic about this right now, but basically I think it's extremely easy to imagine that someone as prickly as Scara would make it to the end of college without fucking anyone. He doesn't like people, he doesn't like being vulnerable, he's horny but not horny enough to get over his myriad hangups & the loner image he's created for himself.
Anyway,
He ends up rooming with Signora (the only person who can stand him) and she throws a little pre-winter break party that freshman hockey player Childe comes to. I am stealing from a TikTok that changed my brain chemistry but basically Scara attends just long enough to grab some cocoa, but while he has the whipped cream Signora makes him squirt some in her mouth and then Childe goes "oh my turn!" And suddenly this huge jock is on his knees in front of Scara with his mouth open (he's short, ok?) and Scara is finally having his sexual awakening and //ohmygodpleasenothim//
Cue Childe following him around like a puppy while Scara has a slowburn identity crisis. He's so mad and awkward about being attracted to this dude that he's especially mean to him to try and make him go away, which Childe reads as him being hard to get.
Eventually Scara snaps and actually goes home with him, but gets a little overwhelmed and leaves before they can get any clothes off or anything. Childe is fine taking it slow, but after a while it becomes a problem because they get very...heated...and Scara always stops before they go all the way. He's literally being edged for WEEKS.
Scara is convinced Childe will get bored and stop pursuing him since he won't give him what he obviously wants...and then one day Scara stays a few minutes too long and touches Childe just right and he just. Comes in his pants. He can't help it, Scara is the most sadistic little Twink he's ever laid eyes on and he just keeps pushing all of Childe's buttons!
So here's Scara, sitting in the lap of the hottest, most popular jock at his school, who just //came untouched// from kissing him and is giving him the most pathetic look he's ever seen, and he's like...I can work with this, actually.
Thus a Dom is born. Scara loves bossing Childe around and being mean and figuring out what he likes and teasing him for it, and Childe adores him and the little games they play. Over time Scara gets comfy touching himself and getting off on watching Childe, and slowly starts to let Childe actually touch him, as a treat, and Childe is even more obsessed with figuring out what he likes/what his tells are.
They get Deep into this dynamic and then one day Scara lets something slip and Childe goes WAIT ARE YOU A VIRGIN??? Oh God I've done this all wrong...
And then Childe insists that Scara have the gayest, most sickly sweet, lovey dovey, cringey First Time™ ever which Scaramouche will swear on his life didn't count because he didn't even like it and he's gotten off with Childe before and virginity is a construct, anyway. But he lets Childe do it because he asks nicely (and if that's the hardest he's ever come then that's between him and God, ok)

Ooh this is interesting! I've thought a bit about it being one or the other but never both at once 🤔
I think there are a few ways it could go, tbh:
1. They each assume the other is very experienced & try to hide their anxiety, fumble around a bit trying to seem experienced, & are carried along by their insane chemistry. They confess the truth in the morning & die of laughter/embarrassment when they realize they could've just admitted it. The next month is spent experimenting & becoming very, very good at getting each other off lmao
2. Somehow they actually manage to talk about this/realize they both haven't had sex before, & take things slow over a week or so. They're both awkward at first, but Lyney is extremely tense & unwilling to say what he already knows he likes/wants to try, & Wriothesley starts to wonder if maybe he's just not someone who enjoys sex.
Ultimately he realizes that Lyney just has a really scary, trauma-related understanding of what intimacy is in general, so he does something radically different from what little experience Lyney does have (spending an hour just touching him & not expecting anything in return maybe?). I could see a service top being awakened when Wriothesley's protective instincts kick in to try & make Lyney comfortable...maybe he comes untouched the first time he actually gets Lyney off...
I feel like I'm gonna end up having a lot more thoughts about this now that you've brought it up tbh

Personality wise, I think pretty much the same! Very sweet and charming, but intensely focused and closed off.
Beyond that, I think he'd be an extremely physical person - he does so much acrobatics and uses a bow in-game, so I always give him a job that involves a lot of movement - and usually performative aspects - in my fics. (Usually I go with things like dancer/pop star because it's a close parallel to magician, but even pornstar or personal trainer works really well imo). I think he likes to feel present and grounded in his body, and that training can give him a sense of control and power he doesn't have in other aspects of his life.
I also think that his canon childhood shaped him to profoundly that a modern version of him would have to have a similar backstory. I always have him and Wriothesley both as former foster kids - just ones who experienced different situations and outcomes (ie juvie vs trafficking + adoption). This sets Lyney up with the same clear reasons for his protectiveness over Lynette and Freminet and his debt to Arle, and can keep the stakes high (if they need to be, otherwise it's easy to tone down what happened to them to fit the vibe). I imagine Lyney would be really passionate about foster care reforms and helping at-risk youth in a modern context, too. Maybe he'd even do some sort of mentorship program for kids in the arts or something! That's a fun way to work the Hearth in in a less child soldier-ish way.
So yeah! Not sure if you were looking for a guide to how I craft modern AUs in general, but there it is, lol.