I personally think that Mifune should have been fighting Madara along side the 5 Kage, but the samurai should at least be at this final battle. If Sakura gets to help, I am sure Mifune would be able to as well.
I'm going to look for that, but it still doesn't explain Mifune not having any thing to say about what is going on.
If Gai and Kakashi get to fight in this battle than Mifune is strong enough to help.
thats a real possibility .......... there is no word on how a samuri would fare on a match up against a gengutsu user !!Or there dead.