I think this post you had made was kinda old but i'm only now getting into the RoL fandom (I'm on Hanging Tree rn). BUT I loved your post about Nightingale's sexuality being nebulous queer for you but also not being central to the books' plot/themes. I totally headcanon him as gay and I think it's mostly because of that first description peter gives him in the first book lol. Confirmation from BA would be fun but I'm also not expecting it ya know?
BA has made comments at Q&As and so on along the lines of “you’re all having too much fun for me to confirm or deny anything” (wrt both Nightingale’s sexuality and the possibility of Nightingale/Mellenby having been a thing), so I’m pretty confident 1) he knows where fanon lies on this issue and 2) he has no intention of bringing up Nightingale’s sexuality unless it’s plot-relevant, which it may or may not ever be. I suspect, without any particular proof, that Nightingale being queer probably wasn’t part of how he originally envisioned the character - but this is a series where characters have evolved as it’s gone along.
But yeah, I’m very much still in the ‘nice but not necessary’ place on this - and that’s down to queer characters actually being a regular part of the series.