What is the significance of this second set of sprinklings? Where did Aharon sprinkle them?
Rashi: This was a second atonement for the Mizbe'ach ha'Ketores, which consists of sprinkling the combined bloods seven times on top of the Mizbe'ach ("Vetiharo" - 'al Taharo shel Mizbe'ach' - Yoma, 59a). 1
See Torah Temiomah, note 85.
What is the difference between "Vetiharo" and "Vekid'sho"?
Rashi: "Vetiharo" means to purify it from its past Tum'ah; "Vekid'sho", to sanctify it for the future. 1
Meaning that the Kohanim should take care not to desecrate it (Sifsei Chachamim).
Why does the Torah refer to "Tum'os B'nei Yisrael", when it is only the Kohanim who have access to the Mizbe'ach ha'Penimi?
Aside from preventing unauthorized persons from entering the Kodesh ha'Kodashim, the Kohanim were also resposible to prevent people who are Tamei from desecrating the Mizbe'ach ha'Penimi and causing it to become Tamei.
