I like the theory but something make me doubt:Hashirama is 100% sure of having killed Madara the great genjutsu user.Hashirama is sure he wasn't in a genjutsu when he saw his friend dead.He could be doubting , but no , he is sure Madara was revived later.
Like Obito , Madara could have been in the other world , and brought back to this world by some help (like obito) or some mental strength(like jiraiya).
Like Obito , Madara could have been in the other world , and brought back to this world by some help (like obito) or some mental strength(like jiraiya).