My impression was that Tuuri was going to find Grandma Ensi among the souls. Possibly something similar as Onni finding Tuuri among the dead. That is what Onni needed Tuuri for, after all.
Also, I don't think Onni was originally planning to defeat the Kade, merely to get Grandma Ensi out of it. Lalli and Reynir being there might have brought a change to that part though.
Even if they do defeat the Kade, they still have Surma, two bears and several wormlike things to deal with; some of them may get to the mages' bodies while they're comatose. There is some hope that Reynir might not go into coma since Icelandic magic is different.