What are the implications of "Chata'im ha'Eileh be'Nafshosam"?
Rashi and Moshav Zekenim: It means that they were sinners in that they caused their own death by quarreling with Hashem. 1
Da'as Zekenim (16:7) and Riva (on 16:6, citing R"M of Kutzi): They were guilty of their own death, because they were warned [that they will die], and did not listen.
Menachos, 52a #1: It implies that their souls were burned but their bodies remained intact. 2
Menachos, 52a #2: It implies that they were Chayav Sereifah because of their souls - because they sinned by flattering Korach. 3
What are "Riku'ei Pachim Tzipuy la'Mizbe'ach"?
Rashi: Beaten out (copper) plates to overlay the copper Mizbe'ach. 1
Rashi: To overlay the copper Mizbe'ach. The Gemara in Menscjhos, 99a learns from the fact that they used the copper pans, which until now had been used to serve the Mizbe'ach, to overlay the Mizbe'ach and become part of it, the principle of "Ma'alin ba'Kdesh' - one goes up in Kedushah. See Torah Temimah, note 5.
What sort of sign did the copper plates display?
Rashi: They served as a reminder that they belonged to the people who had quarreled against the Kehunah and had been burned.