as for the gaiden
minato could only sence it if that special kunai was being used, and kakashi only pulled it out during that last bit, after obito was crushed, that's why minato came so late (also minato had stuff to do at his own battlefield.)
as for now,
orochimaru has no intrest in the war, he said this himself because it's not a war he started by himself.
for the moment I don't think the hokages are going to enter the fight. now their only purpose is to answer sasuke's questions, after that I think oro is going to store them again. his reaserch into the death reaper seal obviously came after his failed attack on konoha and the thirds sacrefice. oro was unaware of the death reaper seal prior to his fight with sarutobi. I presume he did this research in order to get back his arms and his powerfull edo's
the only thing I think oro is going to do with his hokage edo's is test naruto prior to naruto vs sasuke, that is going to be the final fight, but I think naruto vs orochimaru is also comming.
so orochimaru and minato working together, I don't think so, certainly not how you describe minato, willing to sacrefice rin and kakashi so that obito would get his MS ?? highly doubtfull
