They often interact as teacher and student. Luna, with her vast knowledge of the old magic, can teach Twilight things that are not found in modern magic books. For example, Luna might show Twilight how to communicate with the spirits of the night.
Well, the story behind the 'Twilight Music' in 'Twilight Princess' could be tied to the game's plot and the world of twilight. Maybe it plays a role in revealing secrets or triggering certain events.
I'm not sure which novel you're referring to because the name Juushirou isn't particularly famous. If you can provide more information such as the genre, theme, author, plot, etc., I will try my best to answer your questions.
In general, the Twilight Princess manga is regarded as canon. It adheres closely to the key elements and plotlines set in the original franchise. Minor deviations might exist for artistic or narrative purposes, but the core remains consistent.
Their relationship could develop through shared goals. For example, if there is a magical imbalance in Equestria that only their combined magic can fix. As they work towards this goal, they start to understand each other better.