One also has to bear in mind that the Tripitakha was compiled several centuries after the Buddha is said to have lived and has several divergent versions according to the ideological predilections of its editors. Also Pali was probably not the native language of the Buddha. The vernacular of the area where he lived and taught was call Magadhi. (The Jain Agamas are transmitted in a related language called Ardhamagadhi “half Magadhi”) Pali was prevalent in Avanti in West India (the geographical base of Sthaviravadins) not Magadha. The Sarvastivadins used Sanskrit, and Mahasanghikas used yet another language for their version. All of them claimed to have “The Original Teachings of the Buddha.”
So I think attempts to find “fundamental” Buddhism are ultimately futile.
I believe that some scholars actually believe that Pali is an artificial language, a mixture between different dialects that was intelligible to people from different regions.
There is no question that there are some later additions. There are even some contradictions. But by comparing different passages and comparing with the Chinese Agamas, we can get a pretty good idea of what the Buddha actually thought.
Yes I have read that Pali could have been a trade language.
Interestingly this method of textual comparison has also proven useful in tracing the history of the “Hindu” Samkhya school; one of whose early works, lost in Sanskrit, is preserved in the Chinese (Mahayana) Buddhist canon.
Sorry for the late reply but as an introduction I suggest “Encyclopedia of Indian Philosopies Volume IV: Samkhya A Dualistic Tradition in Indian Philosophy” Gerald James Larson & Ramshankar Bhattacharya (editors), Delhi, Motilal Banarsidass, 1987.
The "divergent versions" you refer to are really not so divergent as to exclude the discovery of a fundamental Buddhism. The Chinese collections and the Theravadin are remarkably similar and this despite being worlds apart in language of composition.
So I think attempts to find “fundamental” Buddhism are ultimately futile.