i've just done the emissary quests every day and maxed withered training every run except for the 2 that glitched on me. you need to get 20k into revered instead of 21k for nightfallen exalted since a quest pops at 20k.
i was about 200 rep away from revered with wardens after i had done everything and said fuck it and bought a scroll of ancient knowledge in the AH to see if it would randomly give me warden rep and it worked. the scroll gives you something randomly either AP, +250 rep with a faction, nothing, or exp if youre sub 110. i think you can only use 1 per week.