So the answer to the second question will clarify the first. Basically everyone before Enmebargesi is presumed mythical or unverifiable(Enmebargesi himself is attested in other texts, most notably the Tummal Chronicle and several archaic inscriptions) and the historicity of the early kings is often somewhat sketchy(Gilgamesh for instance is sort of on the fringes of historicitcy). More broadly, the King List is itself an anachronistic document. We have kings we know to have existed not listed in the King List(such as Meskaldamug) and various dynasties we know to have been competing set in linear order; in general the king list can be regarded as a later historiographic attempt to convert a host of competing city-states into a orderly succession of kingship culminating in the scribe's own city(in the case of the Weld-Blundell Prism, the most famous manuscript of the list, the city of Isin). This also clarified why the lifespans were so long-the kings in question existed so to speak in the mythical past and were invoked less to provide a convenient and accurate list of kings then to provide a mythic and heroic charter for the kings who claimed legitimacy based on their place in the King List, so to speak.