The answer to $(1)$ is *no*. In fact, take an irreducible, nodal cubic curve $A \subset \mathbb{P}^2$ and take $10$ points $p_1, \ldots, p_{10}$ on it, different from the node. Let $X$ be the blow-up of $\mathbb{P}^2$ at the points $p_i$ and $C$ the strict transform of $A$ in $X$. Then $C$ is an irreducible nodal curve isomorphic to $A$ and such that $C^2=-1$; in particular, it is isolated in its numerical equivalence class.

This also shows that the answer to $(2)$ is *no*. In fact, the curve $C$ in the example above clearly has non-negative intersection with any curve on $X$ different from it, in particular it has non-negative intersection with all the smooth curves. However, $C$ is *not* nef since $C^2 =-1 <0$.