The Mahāyāna approach, based on the Avataṃsaka Sūtra and others, is that the appearance of Śākyamuni in the world is an expedient manifestation, and that the primary conditions for his awakening are at least fulfilled prior to his birth with the Coronation with All-Knowledge of all Aspects (see Daśabhūmika on the tenth stage). So the manner in which he "displays" himself as awakening in human form is instructive to śrāvakas, but mustn't be taken as definitive to bodhisattvas. In fact, this is partially corroborated on the Śrāvakayāna account, which suggests in the Apadāna and elsewhere that Gotama Buddha needed instructions from countless other buddhas in a Buddhafield, and the merit from offering to them, in order to fulfil Buddhahood in his final birth. The calculation of accumulating merit, in the sense suggested by the Apadāna, again, is an expedient, but in many ways fits the gist of our understanding.
Ultimately, in Shin we see Śākyamuni as a manifestation of Amitābha Buddha. He is an aspect of Amitābha's Dharma Body as Compassionate Means, which gives rise to the nirmāṇakāya emanations. Shinran expresses this in:
Denis Hirota's understanding of shinjin is completely divorced from Shinran's shinjin. If by some great wonder we can overcome our delusions and do good in this life, yes, it is entirely by the working of Amida and we can only be infinitely grateful for this—but this is not connected to shinjin, which itself confers no moral or intellectual transformation. People like Hirota are just addicted to self-power and cannot help but force it back into Shin teachings. The only cure for such people is to read and re-read Shinran and Rennyō. Read through the Kyōgyōshinsho and the Gobunsho 100 times. Then do it again. As Rennyō suggested, if you have heard the letters and scriptures of our tradition constantly and still haven't awakened shinjin then that is due to one's karma.Jōdo Wasan 88 wrote:Amida, who attained Buddhahood in the infinite past,
Full of compassion for foolish beings of the five defilements,
Took the form of Sakyamuni Buddha
And appeared in Gaya