Nicos: Bluff (10+11+6=27)
Medowbrook: Bardic Knowledge (18+13=31), Sense Motive (1+5=6)
Voadam: Sense Motive (19-1=18)
Firen: Sense Motive (12+7=19)
Nicos' actions haven't given anyone reason to suspect that he is not being entirely truthful about his master Retsilah. While you've never heard of Retsilah, his story, as Nicos tells it, is believable.
Medowbrook might consider Nicos' description fails to address the subtleties of Waterdeep, but what he says is generally correct. Nicos does have a record of standing against Xanathar's Guild, though it is kept pretty hush hush.
The diplomats sent to Waterdeep after the end of the elven retreat have since moved on to establish contact with other locations. Medowbrook has a pretty good hunch that a moon elf sailor named Lolaan Cauladra could get word to Amlaruil's Elven Court. He's been known to frequent The Grey Griffon, a tavern in Dock Ward, when he's in port.