I like a name that can be broken down multiple ways to convey things about tone and relationships dynamics beyond the binary divide of formal name and nickname. Indiana Jones jumps to mind; practically no two people in a movie call him the same thing. He's got two different titles, a given name, a surname, a chosen name, a diminutive of the chosen name, and whatever you call the category "Junior" falls into. And no two of those names or name components have the same mood when you hear them next to each other out of context.
I look at Harrison Bach up there and I'm feeling like, out of this cast, he in particular doesn't really give you that freedom to play. Harrison and Bach are equally easy to say, equally serious sounding, and Harry is like the least informal classical nickname. But it is cute that him and his best friend have rhyming nicknames. Me liking the Harry part of it in particular but in light of your discomfort with his sharing the name of Harry Potter, maybe change his given name to Laurence and nickname to Larry? You preserve the rhyme and now all three of his names have a significant tonal difference from each other.
Terry is great, no notes.
Minthe and Minty sound almost exactly the same as each other, so I don't buy Minty as even being her nickname. Minnie or Thee could work better as diminutives but might I suggest breaking the mold here and giving her a nickname that isn't derived from her name at all? I don't have a suggestion for one, though, not knowing anything about the character that might have inspired a nickname.
Adulphine "Alfie" Mordred. In terms of that Indiana Jones factor, these are the three most sonically different names any of these characters have and I love that for her but you're not wrong about it being kind of a lot. The question isn't really whether it's right to be kind of a lot in a vacuum, though, it's whether it makes sense for this character's name to be kind of a lot diegetically.
This curse she carries, what's the nature of it? Is it because she comes from a family of evil aristocrats? Because I have to assume any Mordred who willingly names their kid Adulphine has to be one, even without those names having associations. Frankly, if I met an Adulphine Mordred in real life, my first thought would be "vampires are real" before I registered what other famous people/characters those names sort of/do belong to. If the curse is supposed to be bad luck that could have happened to anybody and her origins are otherwise unassuming, you gotta go back to the drawing board and give her the most incognito formal name out of the whole crew with only an evil-sounding nickname, and a recently-acquired one at that.