You get vectored back later after you go do Underharrow and that other place north (name starts with an M I think, but I forget what it's called). When you return to Edoras, you then get a quest that starts the actual evacuation. The quest description lists some epics and "Edoras: ..." quests that are required in order to advance the evacuation.
As for the letter, I also still have that in my inventory. I'm kinda bummed that we can't really have a house in Edoras. Not sure what to do with the letter.
R5 100 GRD Marevayave - Leader of Riddermarked For Death
R8 100 MNS Fayah/100 LM Siennah/100 HNT Dinenol/102 RK Dhurik
100 CHN Alachas/85 CPT Dinfaerien/60 BUR Dhax/35 WDN Godoric
R9 105 MNS Fayeh (alt Wilya) - Lonely Mountain Band @ Landroval