Who is the "Kohen ha'Mashi'ach"?
Targum Yonasan: He is the Kohen Gadol, who has been anointed with the anointing oil.
What is the significance of the words "l'Ashmas ha'Am"?
Rashi #1 and Rashbam (citing Horayos, 7b): We learn from the Tzibur, where the Torah uses the same expression, that the Kohen Gadol is only subject to the current Korban if (a) he rules that a Chiyuv Kares is permitted and (b) he actually implements his P'sak. 1
Rashi #2 and Seforno: To teach us that, since the people are dependant upon him to atone for them and to pray for them, if he sins, it is due to them that he now became defected. 2
Ramban (on Pasuk 14): To teach us that the same Din applies to the Kohen Gadol as to the Tzibur. 3
Rashbam, Moshav Zekenim: If the Kohen sins by issuing a wrong Pesak, thereby causing the people as well to sin.
Targum Yonasan: It means that the Kohen Gadol sinned by bringing a Korban on behalf of the people not according to the Halachah. 4
Moshav Zekenim: 'Helam Davar Im Shigegas Ma'aseh' means that he erred in deed (e.g. he thought that the piece he eats is Shuman) and was not aware that he sinned.
As Chazal say in B'rachos, 34b - 'If the Shali'ach Tzibur sins, it is a bad sign for those who appointed him' (Seforno).
See Na'ar Yonasan.
What exactly is a "Par ben-Bakar?
Rashi: "Par" implies an old bull (even in its fourth year); "ben", a young one (in its second year) 1 . Therefore "Par ben Bakar" refers to a bull in its third year.
Why does a Kohen Gadol bring an ox?
Moshav Zekenim: Since they sinned with an ox (calf), their Chatas is a calf. And so is Hashem's Midah - the matter He strikes 1 with, He heals with it.
Here, Hashem did not strike with the calf! However, just like He heals with the matter He strikes with, so He heals with the matter people sinned with. (PF)