Hey, Caster (person playing Medea), I read your post and a few things.
1) "No matter, neither of them were worse than your usual combatant in a Grail War, and the enemies here positively paled in comparison to the truly frightening foes in a Grail War."
Incorrect. Mia is actually at the level of some of the fastest servants. And actually moved at the speed to stab Kayneth. You can't just nope this movement, especially the moment she was summoned. You would have to knock Mia away or something because if you interrupt it basically has to be when Mia lands and is literally about to stab him.
2) She was summoned
There is nothing that would allow the summoning in the area. A summoning without a grail can be hand-waved for the sake of the RP, however the fact that she was summoned when there is no magical circle of any kind even present in the area they are in makes it literally impossible and goes a bit to far.
3) You assume success of the wind wall.
Basically as said, you assumed it would work and Mia can't either A) React, B) Dodge, or etc.
4) Her identifying 2 opponents.
There is no way to know this and is not someone as smart as Medea would determine immediately. Neither one is magical and PARIAH does not even have a soul I think, he is just a virus. No reason to view the two as separate entities until evidence presents itself. Also, Pariah is literally the building and not a figure in the hall.
5) Her back was to Kayenth
Hilariously you put her in a situation in which Mia is already about to stab Kayneth in the back, and you don't even have her face Kayneth. Also, again with the assumption. Mia is faster and if they were in a situation where they were facing one another, Mia would just blitz her and stab her. She can do that in the time it would take to raise a hand and say a word, especially because you did not put any mention of the speed of that action.
So ya, here are some problems with the post. There is reacting to the post and then there is completely ignoring my post happened.
Please put more thought into it.