Why does it say "Ya'an Asher Yesharesu Osam"?
Radak: They used to serve Yisrael in front of idols. In place of this, they will serve Me in the future in Avodas ha'Bayis, in matters that Zarim may do. I considered them like Zarim. The entire Parashah explains that Kohanim who strayed [after idolatry] in their lifetimes, this will be their punishment at Techiyas ha'Mesim; "they will bear their shame" (13).
Malbim: Their punishment is Midah k'Neged Midah. Just like they served Yisrael in front of idols, and via them Yisrael stumbled to serve idolatry, they will bear their sin according to their sin. (They will not be gatekeepers, just they will serve the gatekeepers.)
What is "l'Michshol Avon"?
Rashi: Priests to idolatry, to draw them to this.
