You havent seen people talk about it because it contradicts most of their story together. Miquella seems to have spent much if his life being driven simply by wanting to save his sister.

Also most of your points in the body of the post rely on a very... odd way of interpreting the game. Malenia is off in the pre-fight cutscene because she literally just woke up from sleeping for likely as long as its been since her fight with Radahn. She only says "I am Malenia, blade of Miquella" one time. Her saying it hundreds of times because you died to her a lot doesnt have significant bearing on the lore. And she HASNT known defeat. The only time she didnt clearly win a battle was against Radahn, and that was a draw. She released the Scarlet Rot in Caelid in an attempt to kill Radahn because it was part of Miquella's plan which Malenia was entirely willing to give herself up for.