In the OP, you just demonstrated (in shorthand, so it is not clear you grasp the implications of your algebra) that if one inertial observer decides light can get between two events in four-space that an inertial observed in relative motion will also agree that light can travel between the same two events, even when his rulers and clocks don't square with those of the first observer.
Somewhat more generally, if we have
$$\begin{bmatrix}c\,\Delta t' \\ \Delta x' \\ \Delta y' \\ \Delta z' \end{bmatrix}
= \begin{bmatrix}
\gamma&-\frac{\gamma v_x}{c}&-\frac{\gamma v_y}{c}&-\frac{\gamma v_z}{c}\\
-\frac{\gamma v_x}{c} & 1+\frac{(\gamma-1) v_x^2}{v_x^2+v_y^2+v_z^2}&\frac{(\gamma-1) v_x v_y}{v_x^2+v_y^2+v_z^2}&\frac{(\gamma-1) v_x v_z}{v_x^2+v_y^2+v_z^2}\\
-\frac{\gamma v_y}{c}&\frac{(\gamma-1) v_x v_y}{v_x^2+v_y^2+v_z^2}&1+\frac{(\gamma-1) v_y^2}{v_x^2+v_y^2+v_z^2}&\frac{(\gamma-1) v_y v_z}{v_x^2+v_y^2+v_z^2}\\
-\frac{\gamma v_z}{c}&\frac{(\gamma-1) v_x v_z}{v_x^2+v_y^2+v_z^2}&\frac{(\gamma-1) v_y v_z}{v_x^2+v_y^2+v_z^2}&1+\frac{(\gamma-1) v_z^2}{v_x^2+v_y^2+v_z^2} \end{bmatrix}
\begin{bmatrix} c\,\Delta t \\ \Delta x \\ \Delta y \\ \Delta z \end{bmatrix}$$
Then $$\begin{eqnarray}
(c\,\Delta t')^2 - (\Delta x')^2 - (\Delta y')^2 - (\Delta z')^2 & = & ( \gamma c\,\Delta t & - & \frac{\gamma v_x}{c} \Delta x & - & \frac{\gamma v_y}{c} \Delta y & - & \frac{\gamma v_z}{c} \Delta z )^2 \\
& - & ( -\frac{\gamma v_x}{c} c\,\Delta t & + & \Delta x & + & \frac{(\gamma-1) v_x^2}{v_x^2+v_y^2+v_z^2} \Delta x & + &\frac{(\gamma-1) v_x v_y}{v_x^2+v_y^2+v_z^2} \Delta y & + &\frac{(\gamma-1) v_x v_z}{v_x^2+v_y^2+v_z^2} \Delta z )^2 \\
& - & ( -\frac{\gamma v_y}{c}c\,\Delta t & + &\frac{(\gamma-1) v_x v_y}{v_x^2+v_y^2+v_z^2} \Delta x & + & \Delta y & + & \frac{(\gamma-1) v_y^2}{v_x^2+v_y^2+v_z^2} \Delta y & + &\frac{(\gamma-1) v_y v_z}{v_x^2+v_y^2+v_z^2} \Delta z )^2 \\
& - & ( -\frac{\gamma v_z}{c}c\,\Delta t & + &\frac{(\gamma-1) v_x v_z}{v_x^2+v_y^2+v_z^2} \Delta x & + &\frac{(\gamma-1) v_y v_z}{v_x^2+v_y^2+v_z^2} \Delta y & + & \Delta z & + & \frac{(\gamma-1) v_z^2}{v_x^2+v_y^2+v_z^2} \Delta z )^2 \\
& = & ( \gamma^2 (c\,\Delta t)^2 & + & \frac{\gamma^2 v_x^2}{c^2} (\Delta x)^2 & + & \frac{\gamma^2 v_y^2}{c^2} (\Delta y)^2 & + & \frac{\gamma^2 v_z^2}{c^2} (\Delta z)^2 & - & 2 \gamma^2 v_x \Delta t \Delta x & - & 2 \gamma^2 v_y \Delta t \Delta y & - & 2 \gamma^2 v_z \Delta t \Delta z & + & 2 \frac{\gamma^2 v_x v_y}{c^2} \Delta x \Delta y & + & 2 \frac{\gamma^2 v_x v_z}{c^2} \Delta x \Delta z & + & 2 \frac{\gamma^2 v_x v_y}{c^2} \Delta y \Delta z) \\
& - & ( \gamma^2 v_x^2 (\Delta t)^2 & + & \frac{((\gamma-1) v_x^2 + V^2)^2}{V^4} (\Delta x)^2 & + &\frac{(\gamma-1)^2 v_x^2 v_y^2}{V^4} ( \Delta y)^2 & + &\frac{(\gamma-1)^2 v_x^2 v_z^2}{V^4} (\Delta z)^2 & - & 2 \gamma v_x \frac{(\gamma-1) v_x^2 + V^2}{V^2} \Delta t \Delta x & - & 2 \frac{\gamma (\gamma-1) v_x^2 v_y}{V^2} \Delta t \Delta y & - & 2 \frac{\gamma (\gamma-1) v_x^2 v_z}{V^2} \Delta t \Delta z & + & 2 \frac{(\gamma - 1)((\gamma-1) v_x^2 + V^2) v_x v_y}{V^4} \Delta x \Delta y & + & 2 \frac{(\gamma - 1)((\gamma-1) v_x^2 + V^2) v_x v_z}{V^4} \Delta x \Delta z & + & 2 \frac{(\gamma-1)^2 v_x^2 v_y v_z}{V^4} \Delta y \Delta z) \\
& - & ( \gamma^2 v_y^2 (\Delta t)^2 & + &\frac{(\gamma-1)^2 v_x^2 v_y^2}{V^4} ( \Delta x)^2 & + & \frac{((\gamma-1) v_y^2 + V^2)^2}{V^4} (\Delta y)^2 & + &\frac{(\gamma-1)^2 v_y^2 v_z^2}{V^4} (\Delta z)^2 & - & 2 \frac{\gamma (\gamma-1) v_x v_y^2}{V^2} \Delta t \Delta x & - & 2 \gamma v_y \frac{(\gamma-1) v_y^2 + V^2}{V^2} \Delta t \Delta y & - & 2 \frac{\gamma (\gamma-1) v_y^2 v_z}{V^2} \Delta t \Delta z & + & 2 \frac{(\gamma - 1)((\gamma-1) v_y^2 + V^2) v_x v_y}{V^4} \Delta x \Delta y & + & 2 \frac{(\gamma-1)^2 v_x v_y^2 v_z}{V^4} \Delta x \Delta z & + & 2 \frac{(\gamma - 1)((\gamma-1) v_y^2 + V^2) v_y v_z}{V^4} \Delta y \Delta z) \\
& - & ( \gamma^2 v_z^2 (\Delta t)^2 & + & \frac{(\gamma-1)^2 v_x^2 v_z^2}{V^4} ( \Delta x)^2 & + &\frac{(\gamma-1)^2 v_y^2 v_z^2}{V^4} ( \Delta y)^2 & + & \frac{((\gamma-1) v_z^2 + V^2)^2}{V^4} (\Delta z)^2 & - & 2 \frac{\gamma (\gamma-1) v_x v_z^2}{V^2} \Delta t \Delta x & - & 2 \frac{\gamma (\gamma-1) v_y v_z^2}{V^2} \Delta t \Delta y & - & 2 \gamma v_z \frac{(\gamma-1) v_z^2 + V^2}{V^2} \Delta t \Delta z & + & 2 \frac{(\gamma-1)^2 v_x v_y v_z^2}{V^4} \Delta x \Delta y & + & 2 \frac{(\gamma - 1)((\gamma-1) v_z^2 + V^2) v_x v_z}{V^4} \Delta x \Delta z & + & 2 \frac{(\gamma - 1)((\gamma-1) v_z^2 + V^2) v_y v_z}{V^4} \Delta y \Delta z) \\
& = & \gamma^2 & \times & ( 1 & - & \frac{v_x^2}{c^2} & - & \frac{v_x^2}{c^2} & - & \frac{v_x^2}{c^2} ) & \times & (c \, \Delta t)^2 \\
& - & & & ( - \frac{\gamma^2 v_x^2}{c^2} & + & \frac{((\gamma-1) v_x^2 +V^2)^2}{V^4} & + & \frac{(\gamma-1)^2 v_x^2 v_y^2}{V^4} & + & \frac{(\gamma-1)^2 v_x^2 v_z^2}{V^4} ) & \times & (\Delta x)^2 \\
& - & & & ( - \frac{\gamma^2 v_y^2}{c^2} & + & \frac{(\gamma-1)^2 v_x^2 v_y^2}{V^4} & + & \frac{((\gamma-1) v_y^2 + V^2)^2}{V^4} & + & \frac{(\gamma-1)^2 v_y^2 v_z^2}{V^4} ) & \times & (\Delta y)^2 \\
& - & & & ( - \frac{\gamma^2 v_z^2}{c^2} & + & \frac{(\gamma-1) v_x^2 v_z^2 }{V^4} & + & \frac{(\gamma-1)^2 v_y^2 v_z^2}{V^4} & + & \frac{((\gamma-1) v_z^2 +V^2)^2}{V^4} ) & \times & (\Delta z)^2 \\
& + & 2 \gamma v_x & \times & ( - \gamma & + & \frac{(\gamma-1) v_x^2 + V^2}{V^2} & + & \frac{ (\gamma-1) v_y^2}{V^2} & + & \frac{(\gamma-1) v_z^2}{V^2} ) & \times & \Delta t \Delta x \\
& + & 2 \gamma v_y & \times & ( - \gamma & + & \frac{(\gamma-1) v_x^2}{V^2} & + & \frac{ (\gamma-1) v_y^2 + V^2}{V^2} & + & \frac{(\gamma-1) v_z^2}{V^2} ) & \times & \Delta t \Delta y \\
& + & 2 \gamma v_z & \times & ( - \gamma & + & \frac{(\gamma-1) v_x^2}{V^2} & + & \frac{ (\gamma-1) v_y^2}{V^2} & + & \frac{(\gamma-1) v_z^2 + V^2}{V^2} ) & \times & \Delta t \Delta z \\
& + & 2 v_x v_y & \times & ( \frac{\gamma^2}{c^2} & - & \frac{(\gamma - 1)((\gamma-1) v_x^2 + V^2)}{V^4} & - & \frac{(\gamma - 1)((\gamma-1) v_y^2 + V^2)}{V^4} & - & \frac{(\gamma-1)^2 v_z^2}{V^4} ) & \times & \Delta x \Delta y \\
& + & 2 v_x v_z & \times & ( \frac{\gamma^2}{c^2} & - & \frac{(\gamma - 1)((\gamma-1) v_x^2 + V^2)}{V^4} & - & \frac{(\gamma - 1)((\gamma-1) v_y^2 + V^2)}{V^4} & - & \frac{(\gamma-1)^2 v_z^2}{V^4} ) & \times & \Delta x \Delta z \\
& + & 2 v_y v_z & \times & ( \frac{\gamma^2}{c^2} & - & \frac{(\gamma-1)^2 v_x^2}{V^4} & - & \frac{(\gamma - 1)((\gamma-1) v_y^2 + V^2)}{V^4} & - & \frac{(\gamma - 1)((\gamma-1) v_z^2 + V^2)}{V^4} ) & \times & \Delta y \Delta z \\
& = & (c\,\Delta t)^2 - (\Delta x)^2 - (\Delta y)^2 - (\Delta z)^2
\end{eqnarray}$$
Naturally, $$ (c\,\Delta t')^2 - (\Delta x')^2 - (\Delta y')^2 - (\Delta z')^2 = (c\,\Delta t)^2 - (\Delta x)^2 - (\Delta y)^2 - (\Delta z)^2$$ is more powerful than your examples, because it holds for any two events as measured by any two inertial observers, while you take great pains to try to fix one point and time as the origin, use motion only parallel to the x axis, and work only with light.