From what I understood:
1) The whole story happened because evil liese was not trinity seven (as she should), which lead to Selina dying and everything going off-rails and entering a path that would destroy the multiverse, with the only solution being both (or possibly all) Liese's sacrificing themselves to imprison the decaying demon king for all eternity.
2) Aries, as another space-time witch was capable of remembering Liese who is now outside of it, and decided to fix the fundamental issue with the world even if it was already peaceful. By making evil Liese the trinity seven, then the β universe followed exactly the same as the original one, in which not only she didn't exist but the original Liese was never needed.
3) The original Liese in the original universe managed to remember what happened in the β universe, and decided to travel there anyway, to save Aries. There she found out that evil Liese has been the trinity seven all along and Selena was fine (and she did have the memory of the other timeline like the original), so the universe was safe. She created a new Aries based on the memories of the space-time, making so she can live in the universe even though she didn't exist, though that's just a perfect copy and she believes the one who disappeared does exist somewhere outside of the space-time (maybe even in heaven) so she will look for her before leaving (left open ended).
I find it funny how the author in the afterword said he wanted to make a space-time fantasy story and was surprised that it ended up strangely complicated, what did he expect?
Thanks for the translations by the way, I love Liese and really enjoyed all of the spin-off.