I would be hesitant to make the claim that they cannot be revived without a teammate - we revive all of the time without them in non-darkness zones. The Ghost grimoire card even states they can save a guardian from death under the right circumstances (ie no darkness). This begs the question what causes a darkness zone? Could Dredgen Yor create a darkness zone with his weapon of sorrow that did not also damage the ghost? The story is too ambiguous to say anything for certain I suppose, I still choose to believe Yor and Ward are the same individual.