Sellen's quest lets us know those who's fate is crossed with the Carians got a layer of immortality because the stars were haulted.
If Miquella knew this, then he knew his plan is inevitable, he wasn't going anywhere and Radahn's halt is temporary. It was just a matter of time.
Miquella always knew his fate is my belief. He knew what he was supposed to do and also knew it would also fail but really had no other choice. Each step miquella took was a step prewritten in his fate. Each decision he made to circumvent his fate made his fate more inevitable