It's not a 1.4GB download. It's somewhere in the range of 10-15MB, if that. If you click on the Updates tab, it will show the size for the update itself. When clicking back on the games tab, it should show the update size-I don't know why it's showing the installed size instead.
Yes, all updates are independent (okay, almost all).
Impulse is the only platform with the current versions; the versions SDC has are outdated.
Why, specifically, would you want a non-2.0 version of DA or TA?
Do note, 2.0 brings more changes than bug fixes-for one, there's the changed spying system for DA.
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The components (tutorials, movies, etc) should not be in need of an update and in any case the games aren't broken down into components in Impulse, so even if they are in need they will be updated.