Does Skyrim Special Edition include all the DLC?

Yes, it has Dawnguard, Hearthfire, and Dragonborn DLC are already built into the game.

And if you have an account with Bethesda you can also download and use mods however even Bethesda will warn you that mods can effect playing the game. The fact is mods are homebrewed which changes the game some might make minor changes and some will be major changes to the game. With Skyrim there is already issues with glitches while playing the game just by using the mods you are going to have more glitches as I had found this out for myself also enabling mods disables earning trophies/achievements.

Yup.

Everything vanilla ever released for skyrim can all be found on SE. Nothing less, nothing more.

yea, all patches and dlc