Nope
yes
You build a map, save it as a file, put it up for download on any website, and wait for the downloads to start rolling in.
you need the pc version on the computer
A Minecraft account is 100% free. But buying the full version costs 28.95.
No, to play the full version of Minecraft, you need a premium account which has to be done online.
In order to play the full version of Minecraft a license must be purchased at the Minecraft homepage. A free, browser-based version of Minecraft called Minecraft Classic is also available on their website but it lacks the majority of features in the full game.
There is one way, and one way only. You need to find a download containing either a .jar file or the full on .minecraft file of the desired version. Go to %appdata% (When you type it, it will take you to the appdata folder) Then either Delete the current .minecraft or go deeper and replace the .jar with the one you want. Remember to back up newer maps for they can be corrupted with an older minecraft.
You need the full version
If by 'full version' you mean a legal copy (you paid for it), then yes you will.
Minecraftnet is the only place where you can register the premium account that you need to play the full version of Minecraft.
You can play the Minecraft Beta demo by finding the PC Gamer demo on their website. In order to play the full game though, you will need to purchase it from the Minecraft website.
No you do not need to download minecraft classic, its browser based. The full version however does involve files being stored on your PC.