Similarly to your question about the past Buddha’s being male, the EBTs make a clear distinction between a Buddha and someone who has attained bodhi in some other way. While caste and gender have no bearing on the potential for awakening overall in any particular life, a Buddha is someone whose life corresponds to a certain narrative and restores the path of Nibbana for others after it has been lost to the cycles of the universe.
One aspect of that seems to be that a Buddha begins from a prominent position and abandons the maximum amount of privilege possible in their society. This demonstrates the ultimately unsatisfactory nature of worldly attainments, and makes clear that their motives are pure- someone who walks away from another caste or gender role could be accused of gaining a worldly advantage by giving up their prior role. Everyone understands the motives of a menial worker who gives up emptying latrines, or a woman in a patriarchal society who wants to avoid marriage to a domestic tyrant and death in childbirth. The motives of a crown prince renouncing his birth right are harder for the average person to grasp, because they already have what everyone in their society wants.
Primacy among castes seems to have fluctuated among the kshatriyas and brahmins- the EBTs give evidence of this in the Buddha’s debates with brahmins, as do Hindu legends like Axe Wielding Rama, an incarnation of Vishnu who punishes kshatriya for usurping brahmin privilege.
I don’t think that there is a good answer to how old the caste system is. There is at least one verse in the Rg Veda that equates different parts of the primordial being with the four castes- the high castes being the upper parts of the body, and the lower castes being composed of the feet and legs. Some scholars have ascribed the origins of the caste system to the ‘Aryan invasion’ theory of Indian history- that horse-riding invaders from the Central Asia plains made themselves the higher castes, and reduced the natives to menial roles, but I think that theory has been somewhat dismissed over time. Varna may have been a feature of the Indus Valley civilization but we know very little about that era.
The time periods used in the EBTs sound fanciful and hard to equate with historical dating in any reliable way. The time frames involved are described in terms of the cycles of contraction and expansion of the universe, so by that interpretation they come from universes before the Big Bang and current universe, and lived in societies that had reached a stage of development equivalent to that of the Buddha’s historical era. They are from ‘previous versions’ of our world in another universe, so normal historical reasoning doesn’t quite apply.
It’s possible that, as seems to be the case with the Jains, there were some figures from an earlier tradition that were remembered as mythical or semi-mythical characters but without accurate dates for their lives and the intervals between them. Western scholarship tends to dismiss the pre-Gotama Buddhas as fantasy, but I think Xuanxang reported that there was a temple in Bodh Gaya dedicated to Devadatta and the previous two Buddhas, and given the long history of recording lists of kings and teachers it may be possible that some of the names and biographies are those of real figures, but that their dating reflects the transition from history to myth.
The Buddha is depicted knowing about the lives of previous Buddha’s, but this seems to be a by-product of his memory of past births and knowledge of karma. It’s also possible that the names and time frames emerged from visionary/meditative experiences and/or were later inventions to give the Buddha a more impressive-sounding lineage.