>>714512738
>So why did Miquella send his sister to kill Radahn only for her to fail and poison an entire region
I'd say Malenia's scarlet nuke was not part of the plan whatsoever. It goes against everything Miquella wanted for her, and Millicent's quest-line addresses that Malenia had to sacrifice her dignity to do so:

"There is something I must return to Malenia.
The will that was once her own.
The dignity, the sense of self, that allowed her to resist the call of the scarlet rot.
The pride she abandoned, to meet Radahn's measure. "

Radahn was going to die eventually. Miquella visited the aftermath of the war himself, and might have assumed Radahn would succumb to the rot or eventually be offed by the festival.

>charming Radahn into killing himself in the first place and saving himself all that trouble?
Because this isn't how his charm works. It's not puppeteering someone to do his will, it pacifies them. And, even as a literal God, he needs to touch someone to charm them - Radahn would have destroyed him. Personally, I also feel Miquella wanted Radahn willingly as they had vowed.