I can't help but think that Kirena's Void powers would be VERY useful in bypassing this obstacle.
Of course, the longer one is in the void, the more likely one is to encounter one of the Outer Gods. So there is that potential issue.
I mean she could combine Void with the Space/Astral Road/Life boat to minimize the amount of time spent inside the Sea during their advance. Thus they would be able to safety appear much deeper in Hel's territory and surprise the enemy.