If she really time traveled, there should be two Mallorys in 2015 -- the one from the future who arrived to kill Langdon and the one who has been there all along, a young witch who knows nothing. In the voice of Doc Brown, “This is why you don’t introduce a plot with time travel -- because doing it properly is nearly impossible!"
It would have worked if future Mallory dipped into the past
to create this alternate timeline, but then dipped back out only to die in the
tub in 2020, letting the current Mallory live her life, unaware of her other self. Unfortunately, this means Queenie gets stuck in the hotel for all eternity and
Misty and Madison are still trapped in hell. Since a new antichrist was created by Emily and Timothy, the almost forgotten apocalyptic Romeo and
Juliet, it was probably all for naught anyway since this new child will likely
bring it all back to square one and end the world again. Sigh, that was all pointless, then. To quote “Spaceballs,” “Evil
will always win because good is dumb.”