Nasu's take on the canoncity of the routes is that all true endings are canon, just not canon simultaneously, though Arcueid does mention in PLUS-DISC that it's more probable tht the "true heroine" belongs to the Far Side, sdince she and Ciel don't have anything to do wth Shiki normally. So you can't say that her route is the canon route. But she's still the main heroine due to being the titular Moon Princess and her true ending is named Tsukihime.
Arcueid won't be back in her true ending because that defies its own purpose. Nasu only wrote Last Episode because Takeuchi pestered him to do it, for example. And there wouldn't have any reasons for Gaia to create moreTrue Ancestors since Arcueid already does their job just fine.
