Excellent post, and I think you're pretty much spot on plot-wise.
And I fully agree, as I stated in another thread, she'll definitely be back in the next game.
There's no way 343 would kill off the 2nd most important character, hell, actually the main character in some ways. Let alone in the first game of their new trilogy.
There are so many easy ways for her to come back with how Halo 4 concluded. Most of all, 343 spent a large amount of time rendering her and working on the intimate relationship between her and the Chief. That relationship was one of the main aspects of this trilogy for them. I just don't see them completely killing off this idea in the first game.
They worked so far to humanize Chief and build this relationship specifically for this new trilogy, only for her to die in the first game? And the Composer and her wanting to be human aspect will definitely come into play next game.
Not a chance.