Moving across a view axis (the distance does not change):
$$\Delta t' = \frac{\Delta t}{\sqrt{1 - \frac{v^2}{c^2}}}$$
$$\Delta x = 0$$
Attempted translation: "But if the velocity and the x direction are perpendicular, the time-dilation only accumulates."
Attempted translation: "But if original change in x is zero, the time-dilation only accumulates."
Response: Untrue. Please consult my previous post.
Restrict: $$0 < |v| < c$$
Define: $$\Lambda_x (v) = \begin{pmatrix} \frac{1}{\sqrt{1 - \frac{v^2}{c^2}}} & \quad & \frac{-v}{c^2 \sqrt{1 - \frac{v^2}{c^2}}} & 0 & 0 \\ \frac{-v}{\sqrt{1 - \frac{v^2}{c^2}}} & \quad & \frac{1}{\sqrt{1 - \frac{v^2}{c^2}}} & 0 & 0 \\ 0 & \quad & 0 & 1 & 0 \\ 0 & \quad & 0 & 0 & 1\end{pmatrix} $$
Define: $$\Lambda_y (v) = \begin{pmatrix} \frac{1}{\sqrt{1 - \frac{v^2}{c^2}}} & \quad & 0 & \quad & \frac{-v}{c^2 \sqrt{1 - \frac{v^2}{c^2}}} & 0 \\ 0 & \quad & 1 & \quad & 0 & 0 \\ \frac{-v}{\sqrt{1 - \frac{v^2}{c^2}}} & \quad & 0 & \quad & \frac{1}{\sqrt{1 - \frac{v^2}{c^2}}} & 0 \\ 0 & \quad & 0 & \quad & 0 & 1 \end{pmatrix}$$
So $$\Lambda_x (-v) \Lambda_x (v) = \Lambda_x (v) \Lambda_x (-v) = I$$
Likewise $$\Lambda_y (-v) \Lambda_y (v) = \Lambda_y (v) \Lambda_y (-v) = I$$
Likewise for $$\Lambda_z$$.
Also: $$\Lambda_x (-v) \Lambda_y (-v) \Lambda_y (v) \Lambda_x (v) = \Lambda_x (-v) \Lambda_y (v) \Lambda_y (-v) \Lambda_x (v) = \Lambda_x (v) \Lambda_y (-v) \Lambda_y (v) \Lambda_x (-v) = \Lambda_x (v) \Lambda_y (v) \Lambda_y (-v) \Lambda_x (-v) = I$$
This means $$\begin{pmatrix} \Delta t'' \\ \Delta x'' \\ \Delta y'' \\ \Delta z'' \end{pmatrix} = \Lambda_x (-v) \begin{pmatrix} \Delta t' \\ \Delta x' \\ \Delta y' \\ \Delta z' \end{pmatrix}$$ and $$\begin{pmatrix} \Delta t' \\ \Delta x' \\ \Delta y' \\ \Delta z' \end{pmatrix} = \Lambda_x (v) \begin{pmatrix} \Delta t \\ \Delta x \\ \Delta y \\ \Delta z \end{pmatrix}$$ together mean $$\begin{pmatrix} \Delta t'' \\ \Delta x'' \\ \Delta y'' \\ \Delta z'' \end{pmatrix} = \begin{pmatrix} \Delta t \\ \Delta x \\ \Delta y \\ \Delta z \end{pmatrix}$$
Proof: $$\begin{pmatrix} \Delta t'' \\ \Delta x'' \\ \Delta y'' \\ \Delta z'' \end{pmatrix} = \Lambda_x (-v) \begin{pmatrix} \Delta t' \\ \Delta x' \\ \Delta y' \\ \Delta z' \end{pmatrix} = \Lambda_x (-v) \left( \Lambda_x (v) \begin{pmatrix} \Delta t \\ \Delta x \\ \Delta y \\ \Delta z \end{pmatrix} \right) = \left( \Lambda_x (-v) \Lambda_x (v) \right) \begin{pmatrix} \Delta t \\ \Delta x \\ \Delta y \\ \Delta z \end{pmatrix} = I \begin{pmatrix} \Delta t \\ \Delta x \\ \Delta y \\ \Delta z \end{pmatrix} = \begin{pmatrix} \Delta t \\ \Delta x \\ \Delta y \\ \Delta z \end{pmatrix}$$. Even if $$\Delta x = 0$$ or $$\Delta x' = 0$$ we always have $$\Delta t'' = \Delta t$$.
Alternate proof: $$\begin{pmatrix} \Delta t' \\ \Delta x' \\ \Delta y' \\ \Delta z' \end{pmatrix} = \Lambda_x (v) \begin{pmatrix} \Delta t \\ \Delta x \\ \Delta y \\ \Delta z \end{pmatrix} = \begin{pmatrix} \frac{\Delta t - \frac{v}{c^2} \Delta x}{\sqrt{1 - \frac{v^2}{c^2}}} \\ \frac{\Delta x - v \Delta t}{\sqrt{1 - \frac{v^2}{c^2}}} \\ \Delta y \\ \Delta z \end{pmatrix}$$, so $$\begin{pmatrix} \Delta t'' \\ \Delta x'' \\ \Delta y'' \\ \Delta z'' \end{pmatrix} = \Lambda_x (-v) \begin{pmatrix} \Delta t' \\ \Delta x' \\ \Delta y' \\ \Delta z' \end{pmatrix} = \begin{pmatrix} \frac{\Delta t' - \frac{-v}{c^2} \Delta x'}{\sqrt{1 - \frac{v^2}{c^2}}} \\ \frac{\Delta x' - (-v) \Delta t'}{\sqrt{1 - \frac{v^2}{c^2}}} \\ \Delta y' \\ \Delta z' \end{pmatrix} = \begin{pmatrix} \frac{\left( \Delta t - \frac{v}{c^2} \Delta x \right) - \frac{-v}{c^2} \left(\Delta x - v \Delta t \right)}{\sqrt{1 - \frac{v^2}{c^2}}\sqrt{1 - \frac{v^2}{c^2}}} \\ \frac{\left(\Delta x - v \Delta t \right) - (-v) \left( \Delta t - \frac{v}{c^2} \Delta x \right)}{\sqrt{1 - \frac{v^2}{c^2}}\sqrt{1 - \frac{v^2}{c^2}}} \\ \Delta y \\ \Delta z \end{pmatrix} = \begin{pmatrix} \frac{\Delta t - \frac{v}{c^2} \Delta x + \frac{v}{c^2} \Delta x - \frac{v^2}{c^2} \Delta t}{1 - \frac{v^2}{c^2}} \\ \frac{\Delta x - v \Delta t + v \Delta t - \frac{v^2}{c^2} \Delta x}{1 - \frac{v^2}{c^2}} \\ \Delta y \\ \Delta z \end{pmatrix} = \begin{pmatrix} \frac{\Delta t - \frac{v^2}{c^2} \Delta t}{1 - \frac{v^2}{c^2}} \\ \frac{\Delta x - \frac{v^2}{c^2} \Delta x}{1 - \frac{v^2}{c^2}} \\ \Delta y \\ \Delta z \end{pmatrix} = \begin{pmatrix} \Delta t \\ \Delta x \\ \Delta y \\ \Delta z \end{pmatrix}$$