The whole Believe campaign and the Museum of Humanity prove that John and Cortana are never found ( at least not for 50+ years). They're set in the 2600s, and the Chief is mentioned to never have been found. So, he's still MIA, and probably will be for all eternity. Let's not bring him back up.
As for the Keyes-Halsey thing (which has been of admittedly considerable interest to me) I think this is quite plausible, now. I'm shocked and awed, but I accept the great possibility.
The narration at the end is likely Halsey. The voice has the same aged sound as Halsey's, versus the crisper, younger Cortana, plus by that point, Cortana would have faded away, having gone rampant long ago.
MJOLNIR Mk.VII is described in the Halsey diary. Possible appearance in H4?
Regarding my earlier comment on the elaboration of the "All I Need to Know" cutscene: I have played Halo CE countless times. I have that cutscene memorized. I simply wish they'd redone the cutscene in the Reach graphics engine, just because I'd have liked to see it shiny. Because in Halo, there were 2 bones in the face. In Reach, it's mocapped. Also, the animations don't suck.
"Our fighters are mopping up the last of their recon picket..."
-Cortana