i have invedit but my computer keeps saying its a Malware program, but its really helpful. So i recommend downloading it :)
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You can always use an inventory editor. I use INVedit
You must died somewhere else away from your spawn point. You just have to go back to find the dropped item.
Try a mod. You could use InvEdit or SPC (Single Player Commands).
There is no cheat for diamond unless you use INVedit or Too Many Items mod.
No it is not safe.