(1) Solution: From the given information, we can determine that $K$ has coordinates $(-\frac{p}{2}, 0)$. The circle $C: (x-5)^2 + y^2 = 9$ has its center at $C(5, 0)$ with a radius $r = 3$.
Let the intersection of $MN$ with the x-axis be $R$. Due to the symmetry of the circle, we have $|MR| = \frac{3\sqrt{3}}{2}$.
Thus, $|CR| = \frac{3}{2}$,
which implies $|CK| = \frac{|MC|}{\sin\angle MKC} = \frac{|MC|}{\sin\angle CMR} = 6$,
leading to $5 + \frac{p}{2} = 6$. Solving this gives $p = 2$, so the equation of the parabola $E$ is $y^2 = 4x$;
(2) ① Proof: Let the line $AB$ be $x = my + t$, with $A(\frac{y_1^2}{4}, y_1)$ and $B(\frac{y_2^2}{4}, y_2)$,
By substituting into the equation of the parabola, we get $y^2 - 4my - 4t = 0$,
$y_1 + y_2 = 4m$, $y_1y_2 = -4t$,
$\overrightarrow{OA} \cdot \overrightarrow{OB} = \frac{9}{4}$ implies $\frac{y_1^2}{4} \cdot \frac{y_2^2}{4} + y_1y_2 = \frac{9}{4}$,
Solving this gives $y_1y_2 = -18$ or $2$ (discard the latter),
thus, $-4t = -18$, solving for $t$ gives $t = \frac{9}{2}$.
Therefore, $AB$ always passes through the fixed point $Q(\frac{9}{2}, 0)$;
② Solution: From ①, we have $|AB| = \sqrt{1+m^2} |y_2 - y_1| = \sqrt{1+m^2} \cdot \sqrt{16m^2 + 72}$,
Similarly, $|GD| = \sqrt{1+ \frac{1}{m^2}} \cdot \sqrt{\frac{16}{m^2} + 72}$,
Thus, the area of quadrilateral $AGBD$, $S = \frac{1}{2} |AB| \cdot |GD| = 4 \sqrt{[(2+(m^2+ \frac{1}{m^2}))][85+18(m^2+ \frac{1}{m^2})]}$,
Let $m^2+ \frac{1}{m^2} = \mu$ ($\mu \geq 2$), then $S = 4 \sqrt{18\mu^2+121\mu+170}$ is an increasing function of $\mu$,
Thus, when $\mu = 2$, $S$ reaches its minimum value, which is $\boxed{88}$.
This minimum area of quadrilateral $AGBD$ is $88$, achieved only when $m = \pm 1$.