And we have plenty stating otherwise. In fact, of the quotes you've posted, only one specifically mentions Sadow being killed by Nadd and it's from an in-universe journal. Most of the others simply state he goes into stasis which does not preclude the possibilty of Nadd being trained by Sadow's spirit. After all, the Dark Side Sourcebook says Sadow went into suspended animation but then mentions Nadd "awakening Sadow's dark spirit". It's far from cut and dry.