Well considering the 5 girls who talk to Yoda specifically say they learned how to manifest after death one of them, at minimum was the first. Not Qui-gon. Also, the way I took it (and this is just how I thought they were portraying it) was everything up until the final room was trying to pull Yoda away from learning how to become a force ghost. The 5 girls specifically said that up 'till then they had created ALL the illusions Yoda had seen, INCLUDING the Darth Bane ghost. They were constantly testing Yoda to make sure he was worthy of learning the secrets. The Darth Bane Yoda talked to was created by the five girls. It was not a Sith ghost or illusion, thus anything it said cannot be taken as the Sith beliefs themselves and what Yoda says is what he thinks the Sith think.