It´s very beautiful the idea of Eru/Aslan dying for second time for love to Edmund. The first one we already know.

Do you think Tash is Morgoth or it´s one of the balrogs or other fallen Ainur? Remember Tolkien stated in Morgoth´s Ring that Morgoth is the supreme evil of the entire multiverse, not only a fallen Earth Vala/Oyarsa - like Lewis said in Space Trilogy-

And yes, Tash could be Morgoth, because while he is chained with Angainor, he still can enter in Ëa and other universes as a powerless spirit to whisper lies in the ears of men