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
Yes it is.
Anything you want.
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.
Invedit. An Inventory editor.
Yes. you can use INVEdit.
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 Minecraft hasn't updated for you, you may have to force an update. When Minecraft opens, check the force update option and then log in. That should make Minecraft update for you.