What are the ramifications of "u'Min ha'Mikdash Lo Yeitzei"?
Rashi #1: He is not permitted to leave the Avodah in order to follow his parents' coffin.
Rashi #2: If the Kohen Gadol is serving in the Beis-Hamikdash, he does not need to follow his parents' coffin, but may continue to perform the Avodah, even though he is an Onein.
What does the Torah mean when it writes "v'Lo Yechalel Es Mikdash Elokav"?
Ramban, Moshav Zekenim (both citing Bahag) and Seforno: If he abandons the Avodah in order to bury his deceased parents, he will desecrate the Mikdash of Hashem, 4 by demonstrating that he honors them more than he honors the Mikdash and the Kodesh that are under his jurisdiction.
Rashi: Implying that a Kohen Hedyot who serves whilst he is an Onen, desecrates the Avodah.
Ramban: Neither will he desecrate the Avodah, according to this explanation, if he does abandon it to bury the relative (See Ramban).
Ramban, Moshav Zekenim: How much more so will he desecrate the Avodah if he abandons it for no good reason (See Ramban, DH 've'Od Matzinu', who elaborates).
QUESTIONS ON RASHI
Rashi writes that he does not follow the bier. How distant must he be, and what is the reason?
Ramban (according to R. Meir, Sanhedrin 18a): After the procession enters a new place, he enters the place they left. This suffices to remind him not to become Tamei.
Ramban (according to R. Yehudah, Sanhedrin 18a): He does not follow the bier at all, lest amidst grief he touch and become Tamei. 1
Moshav Zekenim (2, citing R. Simchah): Likewise, relatives for whom a Kohen Hedyot may not become Tamei (maternal siblings, a sister who had Nisu'in), he does not follow the bier, lest amidst grief he touch. This is unlike Moshav Zekenim wrote here, that we are not concerned lest a Kohen Hedyot become Tamei for other relatives [if he follows the bier], since he is not frantic about them.