Semicha And Vidduy

 The Sifra Dibbura d'nedavah (5:4) says אין לי אלא של אנשים; של נשים מנין? אין לי אלא של ישראל; של עבדים מנין? עד שאתה מרבה להביא את של עובד אלילים מנין? תלמוד לומר "העולה" – כל שהוא עולה טעונה הפשטה. מה ראית להביאן להפשט ונתוח ולהוציאם מן הסמיכה? אחר שריבה הכתוב מיעט! מפני מה אני מביאם להפשט ונתוח? שהפשט ונתוח כשרים בכל אדם, ומוציא אני הסמיכה שאין הסמיכה אלא בבעלים.  What does the Sifra mean that since smicha has to be done by the owner gentiles are excluded as opposed to flaying an olah?  I think the simple peshat is that smicha is a more narrow din than flaying an olah and therefore does not include gentiles.  The Rabbenu Hillel learns it means since the owner must do semicha he must enter the azarah and a gentile can't enter.  The Rash m'shantz learns that when one does semicha they do vidduy and a gentile does not get atonement from a korban and hence does not do semicha.  [I do not understand how his reason fits in the words of the Sifra.]  The Rash says that in all korbanot including a shelamin there is an element of atonement.  The Ralbag also understands that the semicha is attached to the atonement in his commentary in this week's parsha Ch. 1.  The Keren Orah in Menachot (92a) says that is the reason that  a bechor, maaser and pesach do not require semicha because there is no atonement from them.  The Ralbag already says this idea in Ch. 3.  However, the Rambam Maaseh Korbanot (3:6) says the reason they don't have semicha is מפי השמועה.  The Rambam would seem to disagree with the above yesod.  The Rambam would be forced to disagree l'shitaso in Maaseh Korbanot (3:15) that there is no vidduy recited on a shelamim, yet there is still semicha.  He holds the semicha is independent of the vidduy.  The question on the Ralbag, Rash and Keren Orah is how do they understand why there is semicha on a korban metzorah, nazir etc. that merely serve as a mattir ot eat kodshim?