Baron Harkonnene and Alia
Apparently you missed that section in Dune. Alia was awoken to conciousness in her mother's (Jessica) womb. At that time, she had access to memories that no one of that age should EVER have been subjected to. It slowly drove her mad over the years until the scene in Children of Dune where the Baron tells the other to get out, and manifests itself in her mind. It essentially makes her more insane, but she doesn't hear all of the other voices. Also, she has taken mass quantities of spice to gain a vision of the future, but that just served to plunge her deeper into insanity. The priest Javid also had a role in this, albeit minor. He just encouraged her to continue taking spice and be unfaithful to her husband Duncan (Hayt-Ghola).
To answer your first question, the Baron is preserved in her memories, ala Paul, Leto II, and Ghanima. Through the spice trances, she awoke memories within herself that should have never been there (see above).
As for the "went crazy for the immense pressure of both becoming an Abomination and power?" She went crazy for the simple fact that many of her Ancestors were fighting over her for control of her mind. She didn't have control over the voices, until the Baron offered her silence in return for a place in her mind. She commented that she did in fact kill him, to which the Baron replies that she did. He is merely a memory fragment encapsulated in her mind that she has access to to keep her from hearing the other voices. But, the memory is just as cunning as the real Baron and drives her mad to which she jumps off the high place in the end of the book, commiting suicide.
Theres your explanation. Hope this helps.