I did a course this semester on the black islamic diaspora, and had spent some time looking at the NOI. I found their whole ideology very fascinating, especially in how their mythology seems to have been mashed together from different pieces of theology from various Muslim sects. The Yakub stuff, the stuff about the white devils, I presume those could’ve well been Fard. But the stuff about Fard being Allah and al-Mahdi? His occultation? Elijah Kareem’s flight from the original organisation (Mecca and the Quraysh) to establishing his Nation (Muhammad in Yathrib and his victory over the Meccans)? Those seem to me to be Elijah Kareem/Muhammad’s work. What do we know about how these disparate pieces of NOI theology were pieced together? And who was responsible for which pieces?
If there's any specific theology that you're asking about, I can try my best to elaborate on that further, but I'll start with what you have posted.
I should preface by saying that a lot of the information regarding the Theology's origin will be coming from the voice of Elijah Muhammad himself. The topic Wallace Fard Muhammad is still confounded in mystery, and Elijah, being "inseparable", as himself and other early members of the Nation of Islam described, from W. Fard Muhammad means that information beyond speculation of him and his teachings comes primarily from the teachings of Elijah.
If you want to see what Fard Muhammad taught in the nascent period of the Nation of Islam, "The Supreme Wisdom Lessons By Master Fard Muhammad To His Servant: The Most Honorable Elijah Muhammad For The Lost-Found Nation Of Islam In North America", of which is your best source. This book contains lessons by W. Fard Muhammad that were transcribed and published by Elijah Muhammad for his followers to study and use for themselves.
To put it simply, nearly all of the theology of the Nation of Islam came from W. Fard Muhammad himself, and throughout his 40 years leading the Nation of Islam, Elijah Muhammad would expand on the teachings and make them widely available. There are instructions that Elijah would give, such as to teach and read from the Qur'an once it became readily available in the United States (the Nation of Islam had previously only used the Bible), to observe Ramadan, and incorporated other traditional Muslim practices, but these aren't necessarily ideological. Elijah, in his books and speeches, often prefaces his teachings with "Allah, in the Person of Master Fard Muhammad, has taught me..." indicating that these were things taught to him. Some of the books published by Elijah Muhammad, including Books No. 1 and 2 of "How to Eat to Live" are prefaced by "From God in Person Master Fard Muhammad".
As you speculated, the theology of "White Devils" and the Story of Yacub came from W Fard Muhammad. I've seen the claim that some NOI theology comes from other religions, but I haven't personally seen any proven examples. In most cases, the Nation of Islam takes prophecies or verses from the Bible or Qur'an and applies it to the conditions of Black Americans.
The topic of W Fard Muhammad being Allah in Person is a particularly interesting one, especially because Warith Deen Muhammad, the son of Elijah Muhammad who would eventually attempt to change the Nation of Islam to a Sunni organization, claimed that Fard Muhammad never claimed to be Allah and that his father had innovated it. However, as early as 1942, Elijah Muhammad claimed that Fard did indeed tell him that he was Allah and the Mahdi that the Muslim world was looking for. According to Elijah, “(Fard) said that he was Mahadiah (Mahdi) and that he was Allah who everyone expected to come two thousand years after Christ who was crucified at Jerusalem" in a statement to the FBI in 1942. However, this was allegedly revealed to Elijah alone, which is what brought speculation of the fact to some early followers of the movement.
I recommend using the available online version of "The Supreme Wisdom" to see the specific lessons that were taught by W. Fard Muhammad himself before the disappearance. Hopefully this answers your question - if there are questions regarding certain theological beliefs feel free to let me know.