Mahayana Buddhism summary

I find it interesting the extent to which Mahayana Buddhism can start to look a lot like Christianity the further it gets away from the original teachings of the Buddha. The Buddha left behind a number of basic teachings, including the Four Noble Truths, the Five Aggregates (or the Five Heaps) about human beings, the concept of nirvana, and that enlightenment is a realization of a truth which exists greater than the present existence.
The various schools of Buddhism which arose were almost inevitably because the Buddha did not really leave any instructions for succession after his death.
p. 57 from Buddhism: A Very Short Introduction by Damien Keown, Oxford University Press, New York 1996. (All footnotes will pertain to this text.) This is likely due to the Buddha's suggestion for each person to think for oneself in spiritual matters.
p. 27 “he advised that each person should think for himself on matters of doctrine, cross-referencing views and opinions against the scriptures before deciding whether to accept them. It is a bit ironic that though there does not appear to be any linkage between Buddhism and Christianity, both schools of thought arose during roughly the same time.
The main issue I have with this Mahayana Buddhism is that it seems the original message has become diluted. Now it seems obvious that as time goes on, with any teaching, new understandings will be uncovered and expanded ideas brought about.. This is very clear, and makes sense because it seems particularly inherent to Buddhism that the path to enlightenment continue to be trudged.
However, Mahayana Buddhism introduces many new concepts which contradict the original teachings of the Buddha. For example, there is a Mahayana idea of the Buddha as a being existing simultaneously in three dimensional bodies.
p. 59-60, “which envisaged the Buddha as having 'three bodies' (trikaya) or existing in three dimensions: earthly, heavenly, and transcendent.” This seems excessive in light of the original teachings. First of all, this concept in particular reminds me of the Trinity in Christianity. I think it could be argued that the Buddha's earthly body, heavenly body, and transcendent body, loosely match up with the Son, the Holy Ghost, and the Father. Second, these three bodies appear to contradict the Five Aggregates and nirvana. The Buddha taught that the idea of a continuing self is an illusion.
p. 47, “Specifically, the doctrine makes no mention of a soul or self, understood as an eternal and immutable spiritual essence.” If the Buddha has three bodies, my first thought is that this is three times he is not in the transcendent nirvana, three times he is existing as substantial, which would be a step backward for him. Supposedly, when the Buddha died, he his reincarnation was finished. In this understanding of Mahayana Buddhism, the Buddha is not done with rebirth.
Another dilution of Buddhism's original message appears in the philosophy of service employed by the bodhisattvas. The text describes their work as follows:
“Rather than seeking one's own salvation, in the way the earlier teachings had advised, the Mahayana places great emphasis on working to save others.”
From p. 58
It does not make sense that these bodhisattvas, whom I understand are not enlightened, attempt to lead others to enlightenment. My question is: can a bodhisattva lead to where they have not already been? The first Buddha did not do this.