I even replaced everyone except Ieyasu in the Tokugawa Clan (in order to give that clan the best ronin, right off the bat), and this still worked for me . . . after taking over all but the two Toyotomi provinces, I parked Ieyasu in Owari and made repeated alliances with the Toyotomis until 1614, then just 'Proceed(ed)' until 1615, when the stage started right up.
XBox 360 Gamertag: RougeRogue1