Hey, thank you for taking the time to write down these questions. It's always good to bounce off ideas with someone, so I appreciate it!
So, as currently planned, I'll go with all the named characters being "fully" aromantic and asexual. I've been wanting to write about other ace/aro labels for a while now myself, but for this story, I'm going with this for reasons that are a mix of happenstance and convenience. Huan-Yue (along with one secondary character) is actually an expy heavily based on a character I previously invented for roleplay purposes, and I'm re-using the version of her that was "fully" aro-ace. Based on that, I'm also making Milie the same, because I want to focus on what makes the two main characters different from each other while they share the exact same orientations. For instance, and answering another question at the same time, one difference that will be explored will be their sexual activity.
It's really going to be about how these two very different persons interact, and how they end up connecting and completing each other despite the initial tension between them. It's hard to predict how I'm precisely going to write the details, but I imagine their orientations will be one element among all the things the two characters will exchange about. Both because there's a lot to unpack about the characters besides their orientations, and because I don't want to focus too much on the theme of aceness when I don't think I have first-hand knowledge of it.
(As for why the secondary characters are also "fully" aro-ace, it's happenstance. One is an expy with the same circumstances as Huan-Yue, the other is an android. Also, lately I've been wanting to write a story where "everybody is X". I was initially thinking about "everybody is agender", but as currently planned, this game will do it with two other things instead, including "full" aro-aceness.)
I think this answers the general intent of all the questions?