Who knows? It is Tobi-rama's jutsu after all, maybe he knows how to release it. Also how can you underestimate the 1st when you've never seen him fight properly? He did stand up to madara's EMS and 'win' after all.
eh, if the 4 edo kages fights in the frontline now, that would just ruin everything for me.
cause wtf will naurto and company do now?
so much for that...
"neji, neji, oh neji my neji, i'll avenge you. you're still alive in our hearts."
naruto and company has to finish off madara and tobi.
previous kages, can do something else, possibly with the bijuu or something.
I will tell you what will happen the madara and obito lost control to the juubi and the juubi have another transform which means everyone is fcked up even if all the hokages showed up they still don't have a chance against the juubi.