According to the introduction to K. K. Shastri's The Jaya-Saṃhitā (The Ur-Mahābhārata) much of the Mahābhārata story we know today including the Bhagavad-gītā is a late addition to the text. He writes:
A puzzling problem before us is of godly or super-human characteristics of Kṛṣṇa and Śiva. It is quite easy to find out that wherever such narrations come, they are easily discardable, and their absence is very much helping us in restoring the flow of the narration. They are all inserted at later dates when both of them were established as supreme gods after the expiry of the old Upaniṣadic period.
As for Bhagavad-gītā (23 to 40 adhyāyas in the Bhīṣma-parvan) we are not in a position to include it wholly, or its any part, not only in the Bhārata but perhaps not even in the Mahābhārata.
As for Śānti and Anuśāsana parvas, he says:
In the old text of the Mbh., further additions and interpolations were made up-to the time, when, still, Northern and Southern recensions were not afoot. Though the first entry of Yudhiṣṭhira is found at the end of the 14th adhyāya of the Aśvamedika-parvan, the full achievement of the victory is definitely at the end of the Aśvamedha sacrifice, after which Yudhiṣṭhira enters the city as a samrāṭ. Here it is quite clear that Śānti and Anuśāsana parvans have no place in the Bhārata, most of the matter being of didactic as well as pauraṇika nature as told above.