Here is what I understood (english not my main language):
- Gathering Cold: toggle, while it's active you've a chance to gain Armor of Frost on frost damage taken
- Iced Edge: it activates if you score a critical while Gathering Cold and under Skaldic Rage. Armor of Frost is not related to its mechanic
- The Frozen Fury has no requirements
- Northwind: like Iced Edge, but on vorpal
- Howl of the North: it only requires you to be under Skaldic Rage
So yes, according to me it's a very valuable line
Some additional thoughts:
- The Frozen Fury seems to have very bad DC, I hope it's affected by tactical DC increase otherwise for a STR build it's not worth the cost (maybe it's not worth anyway)
- Northwind provides a very negligible increase in damage output, I would take only first rank to qualify for HotN
- Since for Iced Edge and Northwind you only need to have Gathering Cold active, and there are no relations with Armor of Frost, I would take only first rank, qualifying for everything I need
Hope it helps