For everyone that is stuck in the story.
Progress is made by completing various objectives like your level, lewdness, how far you've upgraded your home, your social progress with the 3 side girls, etc.
To get the true ending you need to complete all dialogues with the 3 side characters (Angelica, Riche and Sylvia). For all scenes to occur you need their NPC favorability at 50 (you can check this stat in your Maidens Handbook).
Finally speak with Angelica to let yourself get checked and there will be an option for an alternative ending.
To open the sealed wall you need to finish the game with the true ending.
The parasitic clitoris tentacle can be found in the Island Outback map in the top right corner near the sealed wall. You need this for the futa scenes.
Scenes can be rewatched by upgrading the basement twice.