I
think, though it's been awhile since I've looked at it, that there's this explanation for Reach's disconnection to the rest of the series (or at least what I took to be an explanation). The Legendary edition of Reach,
maybe the Limited edition too (though I don't know, maybe it was just the Legendary), had this slip of paper that to my recollection, basically said that Reach, the game, was a re-telling of events as best as ONI could piece it together. Now, I could be wrong about this, but I think that's why I said "Oh, makes sense" and didn't really question it.
Now, going on that theory (that Reach was just an imperfect re-telling), I think that the story of Reach does make sense in the context of the universe. If the Covenant attacked only a small part of Reach (which they did, prior to the rest of the fleet dropping in), with only a small group of ships, I doubt that ONI would alert the rest of the population to it. Knowing them, they'd probably do their best to black out any information about the attack, and would only alert those personnel that needed to know about the attack (i.e., a group of Spartans that technically only existed to ONI themselves). Once the rest of the Covenant showed up, that's a different story.
On the topic of Halo 4, I'm liking the look of the S-IVs. Yeah, they're different, but the shoulder lights especially remind me of prior MJOLNIR sets.