For example if x = 0, can I use this card to "negate" 0 point cards?

Definitely. Though spending 2 points for a second Negation is somewhat questionable.

Definitely. Though spending 2 points for a second Negation is somewhat questionable.

Questionable, but still potentially useful, depending on the list.

It would be nice to know more of what the OP had in mind in terms of their list. Unless of course this was a question that came up in a game because someone else played it that way and a commotion was caused...

Yes. There is no trait, so it can be played by any figure (actually, the player in this case), and 0 is a valid number fior friendly spy groups.