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It means, that your account is hacked. delete your .minecraft folder (backup save files) and change your password on minecraft.net. Redownload minecraft and it´ll work.
The end of stream means the server has stopped streaming. This happens when the computer hosting the server is shutdown. It also happens when the server is down for whatever reason.
There is not an End Castle in minecraft, the End is basically only to fight the Ender Dragon. But maybe we will see a Ender Castle in further updates.
Nope
you go on https://minecraft.net/ And it will say buy minecraft