Oh my, the author doesn't let us rest, this freaking Saintess it's gonna be dangerous and I already hate her to the core, just reading what she did to the Princess in the original story gave me chills... geez. >n<
The only way Eclette would be the right heir is if the current Emperor had usurped the throne or if his position weren't accepted as legitimate, but that's not the case (as far as we know); the current Emperor ascended to the throne rightfully and that turns our MC as the legitimate princess as well. It wouldn't even matter if the 'heroine' comes back to the empire, that wouldn't change anything since she wouldn’t have the right to be more than another 'princess' or something similar, but not the heir.
Unless the MC is adopted as well or if this is a matter of mana... also, why does the Princess is the 'heir' if we have a Crown Prince already? Or what does that mean?
Maybe the Saintess really doesn't care about the truth, maybe she's saying all that because the 'heroine' is the daughter of the previous Saintess and that's what matters to her and what she thinks it's 'legitimate', which it could explain why she's so obsessed to help her and destroying every person against the 'heroine' taking the throne. Idk.