Let $S$ be the set of all solutions of the equation $\cos^{-1}(2x) - 2\cos^{-1}\left(\sqrt{1 - x^2}\right) = \pi$, $x \in \left[-\frac{1}{2}, \frac{1}{2}\right]$. Then $\sum_{x \in S} 2\sin^{-1}(x^2 - 1)$ is equal to\\
(1) 0\\
(2) $\frac{-2\pi}{3}$\\
(3) $\pi - \sin^{-1}\frac{\sqrt{3}}{4}$\\
(4) $\pi - 2\sin^{-1}\frac{\sqrt{3}}{4}$