I don't think you can I'm not sure but I'm pretty sure they would have a download link to an updated INVedit
No
Anything you want.
Yes it is.
Almost certainly no.
Open Minecraft and click on the "Option" button and select "Force Update" and it will reinstall the files and start over. It won't touch the saves or world files only the core Minecraft files.
Yes. you can use INVEdit.
Invedit. An Inventory editor.
You can always use an inventory editor. I use INVedit
Download it from from the Minecraft forum. The link is under "Sources and related links" below this answer.
Try a mod. You could use InvEdit or SPC (Single Player Commands).
It's safe.
if you want items in minecraft the best edit i know is INVedit. download it, open world iventory and edit it. save it to your world next