This would be more of a patch type thing than DLC. I don't think any company would force you to buy their DLC just to fix your game.
This. If you mean as alternate means to get patches (ie if you can't connect to the internet), well then, you'd need the Live to get the DLC in the first place anyway, so I don't see a point.
I doubt there would be any other way to get patches until they at least release a GOTY edition.
Again, DLC's wont come with any bug fixes for Vanilla quests. Then only those who bought the DLC would get them. DLC bug fixes will only fix things introduced by the DLC.
Generic bug fixes will come in general patches like 1.6.