How sound is the story of the Companion Sa'd ibn 'Ubada's killing, and is it permissible to discuss it despite the suspicions it raises?

The story you are asking about was not mentioned. However, if it is regarding the jinn killing Sa'd, then it was narrated by Qatadah with a broken chain of transmission, and by Bakkar ibn Muhammad ibn Sirin, from Ibn 'Awf, from Muhammad ibn Sirin. There is no harm in relating such stories, and many jurists have mentioned them. As for how they killed him, Al-'Ayni mentioned that they afflicted him with the evil eye.