I think I have found a shorter way to get under Northrend through Icecrown Citadel, this method doesn't require you go almost to the top of the citadel and find your way down.
Lots of Pictures incoming:
First start out in front of the citadel and head left following the arrow:
Follow the arrow around the edge:
Next Hug the wall, and fly under ICC:
Head/turn left once again:
Fly over towards the small hole in the ground:
Now here there's an invisible wall that is easily overcome, simply fly into it and press your jump key until you fly over, then go into the hole:
Good luck!
I couldn't find anything using this path to get under ICC, however I've seen some use this hole, sorry if it's a repost.