Because of the drugs from the surface, all of the males appear much younger than they really are. Sensei has been in there for quite a long time, possibly hundreds of years and has been an overseer to those girls for a very long time. Kikuchi/Director somehow underwent so much shock after what happened (I guess it was from Sensei's beatings offscreen? Idk) that his true form was shown; as an old man with the mind of a child. Youko and Sakura are adults at the end, and Liz is Sakura's daughter who most likely came from the original Liz while Youko has been taking care of Kikuchi/Director. Just like Minmin's death, they all cope with it and move on. But they're all stuck down there now and the surface must really be screwed over if they can't get any more suitable mothers from underground. And since they're just humanoid Guardians, it won't be long until Youko and Sakura pass away. The origin of the Guardians and the reason both of the genders have been separated is beyond me, but that's my view of the ending.
The yuri died with Minmin. Not too sure why I kept reading.