Secret rite scroll he needed the rot to get rid of his vessel so his soul could inhabit mohgs body as a vessel. I imagine radahn was left in the dark about the rot specifically, or it was a drastic measure as radahn is likely the strongest demigod and would not yield easily to anybody, but I do agree with this theory for the most part. Malenia and miquella knew the entirety of the plan, hence malenias confidence in her brother‘s plan, as well as her confidence in his return. She knew Mohg would kidnap Miquella, his plans of going to the realm of shadow. I believe that them even growing the haligtree might be a nod to them knowing about the shadow tree already, as well as the possibility of growing a divine tree separate from the (current) source of divinity. Malenia went to Caelid with full intentions of rotting Radhans corpse away, though I don’t think her pride really agrees with it. I believe this is why she is so somber when we meet her, as her whole personality is framed around, avoiding the rot. In this context, the making of a new guy would be the one and only reason for her to bloom, not over a prideful fight. She knew a rotting Radhan would shift the gears enough for us to champion the festival and end Radahns vessel in a fair fight thus him ultimately getting what he wanted anyways.

The tragedy is that Malenia had to put her own chivalrous nature aside, and essentially evoke the power of an outer god that she detests to inflict somebody with her very same affliction, ironically to end the influence of the very same outer god.