What happens in the Ainulindale in the "TimelessHalls" and what happens afterwards in Ea .  Unlike Paradise Lost, Tolkien did not mean for the Timeless Halls and Ea to be energetically continuous.  Ea is not a subsystem of the Timeless halls or embedded in it in any meaningful way.  The idea of timelessness makes it incomprehensible.  It is a myth of information, of the way collaborative efforts can go wrong when someone refuses to communicate.  Tolkien himself called the Ainulindale abstract and treated it as a discontinous entity from the rest of history, in fact, it can be argued the "events" of the Ainulindale are not part of the history, because they happen outside of time and space.  Also the Ainulindale is more of a ritualized or abstract version of Paradise Lost than Genesis.  I think the Ainulindale, the second version at least, is a very beautiful text, but it is not meant to be taken as literally as the later myths, and if you asked Tolkien what he meant by it, he would probably say well everyone want's to know how it all began and this is the best way the Valar could tell the elves and the elves could tells human what came "before" (In fact he does say this in Morgoth's Ring). The most literal of Tolkien's narratives is of course The Lord of the Rings, since it is actually written from first person accounts, the hobbits.

The difference between Eden and the Spring of Arda is that Eden maintained a set level of complexity which was the optimal level for creation and the status quo until the fall.  The complexity in Ea increases until the Dagor Bragollach and the breaking of Arda.  This is amazingly important.  It is similar to Odin killing his father Ymir and making the world from his body and ushering a time of optimal peace that has sense passed or the golden age in Greek mythology after Pontos, Gaia, and Uranos  evolved from chaos, and Cronos kills his father and before he goes mad and starts eating his children, or when Enki kills Apsu and then Marduk kills Tiamat and recovers the tablets of creation in the Enuma Elish.  The Valar as subcreational powers are analgous to spontaneous organization.  For Tolkien, increase in complexity has to be driven by a conscious mind.   This is very important, because it is one of the ways in which the history of Ea is differentiated from Christian myth and is more like these older pagan myths.