Nadia:-
Perfect. If you start one, I'll do the other. But I think we should start with Pavo/Darian before Hogwarts closes for the summer, so do you want to do that one or shall I?
Addie:-
I'm not sure about the odds of Phaedra taking Barnaby under her wing even if she did pity him. However, I can absolutely see her criticizing Frank's treatment of him if it were lacking, probably more as an excuse to tell him off rather than due to a real concern for Barnaby (allegedly). But it is entirely plausible that she'd be like "yo, if this dude is gonna stay then you need to step up and take care of him or I'll take care of you" or something. Or even just calling him out for being a general butt. I will say though, depending on how grating Frank is, she might be induced to help Barnaby just to show Frank up. She definitely already pities him and did feel some concern over his involvement in the heist, so I guess it depends how helpless he seems and how unappreciative Frank is.
I don't think it's unlikely she'd listen to Keiran's concerns if he needed to air them. Given that she's seen his memories and thus has an (I'd assume somewhat decent) understanding of his relationship with Millie, she might have insights to offer on that end if he were receptive (and if she managed not to offend him long enough lol). Or just a sounding board for any woes. I guess it depends what you had in mind, but it sounds viable.
Flourish & Blotts is a completely plausible destination for Phaedra, so I can easily find a reason to send her in to meet Frank & Barnaby &/or Keiran. I'm equally happy to put her in other locations, so wherever you think your lot are likely to go I can probably conjure an excuse.
As for Phae/Lucien- I'm currently working on posts for Nadia and Khaat so if you're up for starting it I can get to it soon-ish. Otherwise, I can work on a starter once the others are seen to?