Just to be clear regarding my memory of Halo 3's ending...after the slip space portal closed leaving MC and Cortana's half of the Forward Unto Dawn drifting outside of the galaxy, did it just happen to casually float towards Requiem - a planet apparently right by the Ark? If you don't recall just how far away the Ark actually was relative to the Milky Way Galaxy:
They made it through the portal and away from the ark (otherwise Chief would have been killed in the Halo blast), it collapsed when they went through putting them somewhere seemingly random - next to Requiem.