I'm not sure what you mean by this. You don't encounter N'zoth until you get inside the raid. Meanwhile, you can experience the corruption effect, including gears that can corrupt, before you even do the 8.3 starting quest that gets you the cloak. In fact, in the beginning, people in LFR can even go to N'zoth without getting the cloak before Blizzard fixed it eventually.
I think they just wanted to redo quests and add flying to the original zones and they used Deathwing as lore reason, so the quests had to reflect that even though they're at low levels. If they didn't tie it to Deathwing, they wouldn't even need the Cataclysm itself and just have Deathwing be another threat without affecting the landscape.Now, that particular example doesn't have the presence that the TC was looking for, as it was only ever meant to be a story-immersion component rather than sweeping change, but the same could as easily have occurred for Cataclysm, bringing players through what parts they wanted of the original experience and then reopening the original world through its changes after Deathwing's awakening.
You have to be careful with the part in bold.It just depends on the changes one wishes to make and what range they wish to affect. The "sweeping change" could occur at endgame, and yet provide excuse for improvements at endgame and so forth that in turn provide excuse to make general improvements to the leveling experience, all without retroactively touching the game's storylines in any way.
For example, you can change the FATE system in this game and tie it to an event that happens at MSQ level 80, but if you do that, it does not make any sense for that change to be experienced by people who have not done that quest. Or, you can just change the FATE system without tying it to any particular event and make its effect felt throughout the whole game.
Anyways, after rereading the OP, I hope that the roulette for tomestones isn't going to change. The point is to give max level players a reason to do old duties and help others queue, so I'm not sure how a calamity is supposed to change that while still helping out the queue.