Why did he say "Mizbe'ach" twice?
Rashi (from Tanchuma Toldos 12): He addresses the one in Beis Kel, and the one in Dan. 2. Radak: It is normal to do so when calling someone, e.g. "Avraham Avraham", "Yakov Yakov" (Bereishis 22:11, 46:2), "Moshe Moshe" (Shemos 3:4), "Shmuel Shmuel" (Shmuel I, 3:10).
Was Yoshiyah already born?! His son (or grandson) was king at the time of the Churban (about 370 years later)!
Radak: Here, Nolad means that he will be born.
Malbim citing Mahari: The Navi perceived with such clarity as if he was already born.
Why does it say "Atzmos Adam"?
Radak: Now, animal bones (of Olos) are burned on you. Later, human bones will be burned on you. We find that Yoshiyah did so.
Radak citing Tanchuma Ki Sisa 6: This refers to Yaravam's bones. Amidst honor of the king, he did not explicitly say so.
Malbim: He will burn [live Kohanei ha'Bamos,] the bones of those who already died, and also Yaravam's bones will be burned.
