"is facilitated by a series of artifacts that contain encoded star charts. The one on Reach led to Earth. The one on Earth, buried somewhere in Mombassa, led to Halo 05. That one may or may not either reveal the location of the Ark, or it may have been within the Ark itself."

That's not quite right - the artifact with the co-ordinates for Halo 04 was found on Cote D'azur in 'The Fall of Reach', and Cortana used the co-ordinates found there to escape to Halo as a calculated blind jump. The artifact on Reach was destroyed at the end of First Strike, without the Covenant getting a look at it. An artifact was found floating in space by the Apocalypso in ILB, and taken to Earth, where it emitted a signal that the Covenant followed to find Earth - the artifact was being held at a facility in New Mombasa, as told in the last few chapters of the ILB story, so the Covenant landed there to try and retrieve it.

It seems that the Prophets already know the location of all the Halos. Truth states in the cutscene where the Brutes take over, that they have known about the rings for ages, but not how to use them. The events of Halo 1, and the discovery of 343GS, educated them in how to use the Halos.

The biggest question is: Why did Regret come to Earth? It seems he followed the Artifact signal to New Mombasa, did not expect it to be the Human homeworld, used tactics learned at Reach to bypass the orbital MAC guns to get to the surface, get the artifact (presumably) and escape to Halo 05, but WHY he needed the artifact, and how the artifacts fit into the storyline, is unclear. Perhaps they hope the artifacts tell how to use the Halos, but on the discovery of 343GS they no longer need to look for answers...

What Regret is doing on Halo 05 is unclear, perhaps he is trying to use his artifact from Earth to power up the Halo?
Mayyybe, the shrine the Regret is at is Halo 05's Silent Cartographer, and Regret is looking up the location of Halo 05's control room.

Cortana says that the Covenant fleet arrives, and is targeting the installation where you and Regret are, but is sending Phantoms to intercept you before you reach Regret. As soon as you kill him, the Phantoms are withdrawn and they blast the installation. This is reiterated by Spec-ops at the beginning of the next cutscene: "If you had not withdrawn our Phantoms, the Demon would have been in our grasp!"

I'm not sure about Cortana's motives for staying on the High Charity either. It's conceivable that she hopes the explosion from the In Amber Clad would take out the High Charity and Halo 05, but it's a bit fishy. Also, why/how is Gravemind on the High Charity at the end?

It seems pretty clear that the Forerunner are actually Humans, and this would mean that the Ark is: Earth. Or on Earth at least. The only place after the activation of the Halos that Human life survived... Hence why Truth leave for Earth at the end.

NiceMissMayonnaise