I suppose that's a retcon to "they just sent some random guy because the Demon Queen is a joke" gag at the beginning. No, they didn't. Soil apparently took this entirely seriously and sent their greatest butcher. I guess they always intended for Mirio to bring down the Kingdom after the Demon...
Once again, the writing doesn't make it super clear, but the flow of time in Mirio's world is convoluted (where have you heard that one?) and everyone present has been there since the beginning of time. That is, they've been there since the beginning of the world. Life is evolving and...
You can simplify this much more. The thrust of what she was saying wasn't really defined by the magibabble, it was mostly implied. She made the micro-world they're in, and she got to define the rules of physics. She spends a lot of time explaining the rules, but the important part is that she...
It's fine that it's ending here. I would have loved a few more story arcs, but I also understand that the author might have been running out of steam in terms of actual writing. I hope this isn't the last we see of Mira, one way or another.