I have read on the wiki that the quest Caesar's Favor actives if you are accepted by the Legion for a while but I have been Idolized for most of the game. I'm asking if the quest activates if you have been to the Fort or if you just have to be accepted. P.S. When I fast traveled to Camp Searchlight, someone called Disguised Frumentarius was killed by the NCR troops, so does that person's death mean that I can't get Caesar's Favor because I want some rewards

The short answer is that I'm not sure.

According to the wiki Frumentarius is indeed the quest giver, but I don’t know if some one else will if he is dead. He's also the one who will invite you to Caesars fortress after you first deal with Benny, but if he’s already dead (I shot him) then another guy does instead.

Here's the wiki page... http://fallout.wikia.com/wiki/Caesar%27s_Favor

I'm pretty sure noone else will come.

However, if you're on PC you can start the quest trough the console - not entirely sure which command though.

**** NCR!!! Now I'll never get my rewards :*(