You can still add mods using a different launcher through Demo/Single player you just can't join multiplayer servers. You have to purchase a premium account for that.
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No
Go to start menu, type in %appdata%, if there is ".minecraft" when you search, then you can. Anyways, why using demo?
bearshare is not required to get minecraft mods.
download it then chuck its contents in a mods folder in minecraft .jar
no