Yes. you can use INVEdit.
No
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.
You can't turn off the timer since it's a demo game.
You use a hoe on grass and seeds come out and you plant those for Wheat
You can't. There are two Minecraft demos that I know of: one on minecraft.net, and the PC Gamer demo. Both are locked in survival mode, and in survival mode there is no way to spawn mobs.
If you are referring to the PC Gamer demo version, yes. If you are referring to the Classic version, no. It is very limited, and meant to be just for building.
you can
Sure is!
Go to minecraft.net and click "Buy Minecraft" or "Play the Demo".
I think TFC's mod manager can run on the pc, but that's all that i know
Morrowind, Minecraft
There is no way to legally play the full version of Minecraft for free. But an issue of a Magazine called PC Gamer had a free demo of an older version. Also, you can play the Classic version for free at Minecraft.Net