What sort of Ketores did Moshe instruct them to bring?
Rashi (on Pasuk 6), Targum Onkelos and Targum Yonasan: It was the morning Ketores of Mitzvah.
Who were the "B'nei Levi"?
Seforno and R. Chananel 3 : They are the 250 men.
Rashi: Moshe began speaking to Korach softly; but when he saw that he was digging in his heels, he included the rest of the tribe in the warning, before they too, joined the rebellion.
Ramban: Although Korach argued on behalf of the Bechoros (Refer to 16:1:9:1), that was only a sham - to make the people believe that he was arguing on their behalf - whereas in reality, all he wanted was the Kehunah Gedolah. Consequently, Moshe, who saw through his sham, admonished him that he should be satisfied with the Kavod allotted to the tribe of Levi.
Korach was astute! Why did he enter this folly, to rebel against Moshe?
Rashi citing Tanchuma: He saw (with Ru'ach ha'Kodesh) a great chain that would descend from him
It is not clear however, how his sons were initially destined to die, seeing as they are not listed among the plotters