Yes absolutely. Some of them will require you to do an update or seperate patch to support Dawnguard, but any ENB or graphical mod that doesn't change textures or interior lighting should work fine.
Some of them. If they change certain parts of the Skyrim world, like making the rivers' shoreline or make shadows look better, then the changes will not affect the new zones. Usually, the old zones will still work fine though.
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