My own headcanon basing itself a little on the old PM to justify and fix the reason behind the Mélanie from the new PM
That's not her in the doombuggy in the end. She acts too differently from how she did during the whole ride waiting, guiding us and crying. Now she is menacing and laughing evilly like crazy, nagging even and somewhat trying to tempt us
But keep in mind that right before, "hitchhiking" us is the Phantom. Like he did in the old ride when he was corpse-looking while he was skeleton-looking before... and was the one with us in the doombuggy afterward
What if he can shapeshift since he's a ghost ?
And the Mélanie in the end is him, both to mock her as well as to scare us away from her so we won't help her ?
That could be in-character and explain things all while keeping the real Mélanie untouched and enhance how he is indirectly controlling her at all times everywhere and she resigned (now he is behind her on the ballroom balcony and in her mirror's reflection haunting her/watching her) and why we don't see her guide us out anymore, because we *avoid* her after what we saw and who we thought to he her, plus now he "guides" us out so that he can keep his prey for himself forever
Plus who knows, since after all he was already a ghost when he killed her fiancé on the day of their wedding, mayne he lured him in by passing as her
"She" even looks scarier/different from what she in the old ride now, with greyish skin and pronounced black eyeshadow as if he wanted to pretend she was turning scary like a banshee and truly scare us
Furthermore, in a way, we tend to forget that maybe it was already the case in the old ride : in the end of it, we would see a doll-sized Mélanie sounding more evil and telling us in a mocking/nagging/tempting way to hurry back because she was dying in loneliness which completely contradicts the whole purpose she had for the whole ride helping us out. So what if even back then it was the Phantom under her traits, and he could even shrink and did so so he could fit in the wine holder and scare us with surprise, back then wanting us to stay so that he could trap us or scare us away too all while mocking her ?
I don't think he possessed his own body back when we saw his corpse self, even with the coffin at his side, I take it as him being extra to scare us even more
That could also explain why he now has red pupils and other times not. He chooses to depending of his mood and level of scary
In short : the Phantom can change size and looks and look like other people, and uses that power to forever keep his daughter miserable