How can the Prophet be a "nadhir" (warner/ Nadhir) to the believers, when warning is for disbelievers, and is it possible that the meaning of "nadhiran" in the verse is (mandhooran/ one who is vowed) not (mundhiran/ warner)?

Believers need and benefit from warning, for they are the ones who fear its consequences and act and obey. Two interpretations have been mentioned regarding the verse: "I am only a warner and a bringer of glad tidings for a people who believe." The first is that believers are the ones who benefit from both warning and glad tidings. The second is that "warner" relates to an omitted phrase, estimated as "for the disbelievers," and it was omitted because it is well-known.