The path at the start of the fjords can be used to safely return to Broken Valley by foot. You will not get very far down the path on the Broken Valley side, though.
In D2:ED the order of the flying fortress from easiest to hardest was Stone, Raze, Broken Valley, Kali and then Keara.
The sigil isn't needed until you are ready to head to the Hall of Echoes, so the order doesn't matter much (you still have to finish off everything you want to do in Broken Valley, the fjords and Sentinel Island first).
