To your first question, the answer is generally no.
The matrix
$$
N= \frac 23 \pmatrix{1&1\\1&-1}
$$
will have (Euclidean) operator norm $4/3 > 1$.
$(I - N)^{-1}$ will exists if $\rho(N)<1$ ($\rho(N)$ denotes the spectral radius of $N$). This in turn is true if and only if $\|N\| < 1$ for some multiplicative matrix norm $\|\cdot \|$.
To your first question, the answer is generally no.
The matrix $$ N= \frac 23 \pmatrix{1&1\\1&-1} $$ will have (Euclidean) operator norm $4/3 > 1$.
$(I - N)^{-1}$ will exists if $\rho(N)<1$ ($\rho(N)$ denotes the spectral radius of $N$). This in turn is true if and only if $\|N\| < 1$ for some multiplicative matrix norm $\|\cdot \|$.