You can't without mods.
You can acquire the Skeleton Key through the console command player.additem 0003a070 1. Source: Elder Scrolls Wiki page "Skeleton Key (Skyrim)"
Skyrim for Pimps - 2011 Quest for the Skeleton Horse 3-5 was released on: USA: 24 August 2012
0003a070
Yes, there are werewolves in Skyrim, and the player is able to become one.
There are no classes in Skyrim, a character is free to develop and become proficient in whichever skills they wish.
You can't but you can get the Skeleton Key as part of Thieves Guild quests which is a unbreakable lockpick!
I don't believe you can summon a skeleton out of thin air like an atronach or familiar, but if there is a dead skeleton, you can use the Raise Zombie or Reanimate Corpse spells, and it will revive and fight for you.
No.
You have to buy the DLC
You cannot become a Werebear in Skyrim, unless you've added a mod that gives you that option. The only were-creature the base game offers, is Werewolf.
sometimes you can turn into a skeleton by eating something
No. When you have completed the quests, there is no way of getting it back!