Let $\Gamma(N)$ be the principal congruence subgroup of level $N$ in $SL_n(\mathbf{Z})$, where $n\geq 3$. Then $\Gamma(N)$ is residually $p$-finite for all primes $p$ dividing $N$.

Can $\Gamma(N)$ be residually $p$-finite for a prime $p$ which does not divide $N$ ?

On a related note: $\Gamma(N)$ is residually $p$-finite for only finitely many primes $p$. The proof I know is somewhat indirect: 1) (Rhemtulla) if a group is residually $p$-finite for infinitely many primes $p$, then it is orderable. 2) (Witte) no finite index subgroup of $SL_n(\mathbf{Z})$, where $n\geq 3$, is orderable. Is there a more direct / hands-on proof?