If they've all been removed because you completely removed Minecraft from your computer and deleted the minecraft folder, then there's no way to get them back unless you have a backup of the minecraft folder saved somewhere else. If you have a backup just copy the texturepacks folder across. Sorry though, there's no way I know of to get your servers back.
Well I dont think you can you would have to search on google for a download sorry.
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.
You may have corrupted the world by using mods or other unlisenced programs to edit the game. It is best to revert to a backup that you have saved before.
Yes there is such thing as the Ether World in Minecraft.
Get a ghast to shoot it's fireball into the nether portal and if the portal is gone and you have no obsidian you are stuck and must make a new world, thats why you should always make a backup
Yes it does, i have it myself. Be sure to backup minecraft and delete META-INF Or even watch a tutorial. Have fun!
An underwater world seed for Minecraft is used to open an underwater world to play the game in.
Sorry it's probaly not possible ):
Yes, Minecraft is global.
No
Place the world in your minecraft bin's saves folder.