Early Buddhists didn't refer to themselves as Theravadans or Mahayanans - those distinctions, as is the case with Catholic v. Orthodox Christians, Sunni v. Shi'i Muslims, and so forth, are historical artifacts. However, as is the case with Neopagans, an attempt to live a pure 'Early Buddhist' approach is equally untenable - the Nikayas/Agamas are themselves historically situated, and while much likely goes back to very near the Buddha's parinibbana, certainly not all of it can be said to do so.
The flavor of the Dhamma, however, is in my estimation still available. In The Questions of King Milinda, which is a late part of the Kuddakha Nikaya, the king asks how the Dhamma can still be said to exist even if the precise words of the original Buddha have been lost. The response uses the simile of a reservoir - even though the original water filling the reservoir has since flowed on, the water has been continually refilled by the Noble Sangha. Indeed, having access to the Nikayas et al in translation has been a priceless boon to me in making the Dhamma a life practice.
My question is twofold, and revolves around the following list:
(1) Are there any extant Dhamma texts not included here (barring Chinese equivalents such as the Samyuktagama) which are considered to be substantially earlier than those in the list?
(2) Including any works taken to answer the first question, which part of the Dhamma is not contained in these texts? Alternatively, which part of the Dhamma cannot be understood solely by reference to these texts?