Nazir 47a - Nazir and Kohen Gadol, Reducing the Issur

Both a nazir and kohen gadol (kg) can be metameh for a meis mitzvah, but there is an argument in the mishna what should be done when they are walking together and chance upon a meis mitzvah. R' eliezer says that the kg should be metameh because he doesn't need a korban for his tu'mah, and the chachamim say the nazir should be metamei because he doesn't have kedushas o'lam (Tosafos seems to understand that even if he is a nazir o'lam, the concept of nezirus is limited to 30 days, so even when he extends it forever it isn't as powerful as kg which is inherently a kedushas olam. See also Rashash who seems to understand that it means that it is no'heg l'doros).

The Maharatz Chiyus asks in the name of the Sha'ar Hamelech: The Ran (yoma 4b b'dapei harif) explains that when a choleh needs to eat on shabbos it is preferable to shecht for him to violate the issur shabbos once, rather than to feed him neveila and violate the l'av of neveila on each kezayis that he needs to eat. Based on the Rambam (Nazir 5:21) that a nazir who deliberately makes himself tamei is in violation of 4 l'avin, it should be preferable to have the kg be metamei a more chamur issur once rather than the nazir violate 4 issurim. So why to the rabbonon feel that the kedushas olam of the kg requires the nazir to become tamei rather than the kg?

Perhaps the Ran's distinction only applies to a case where he will be doing multiple actions to violate the multiple issurim, such as eating one kezayis after another. But in a situation where he will be doing one action that violates a more chamur issur i.e. kg being metamei, or one action that violates many issurim in one shot i.e. a nazir being metamei, it is better for the nazir to be metamei.

But to me it seems that the real answer is: When we deal with violating issurim and the rule of thumb is "ha'kal ha'kal techila", meaning we reduce the level of issur, the Ran suggests that reducing the amount of issurim is more important than reducing the severity of the issur. Meaning, that one issur chamur is more "kal" than many lower level issurim. That concept only applies when we are basing our approach on "ha'kal ha'kal techila". However, when it comes to a nazir and kg who are in conflict which one should violate his kedusha to be metamei for a meis mitzvah, we don't look at the severity of issur, rather we look at their respective levels of kedusha. The individual with the higher level of kedusha can demand that the other be metamei, even if by doing so he will be in violation of more issurim. Therefore, the Chachamim hold that the kedushas olam of the kg gives him the upper hand to force the nazir to be metamei.