NSA or any other agency won't gain anything by destroying Bitcoin because it's not a threat to the national security of any country right now and I don't think it will be any time soon. Plus Bitcoin is open-source right? So any tampering with the source code or placing backdoors would be easily noticed wouldn't it?
Abslutley! it would be an exercise in futility for any government to try and manipulate bitcoin. The genie is out of the bottle nad now we are in control.