= Solution
Apply the same symmetric-ejection and <causality> argument to the launch <proper motions>, when the proposed ejecta are fastest. One obtains
$$
x_0=\frac{101-59}{101+59}=0.2625,
$$
and
$$
\boxed{\theta_{\max}=\arccos(0.2625)=74.78^\circ,\qquad
D_{\max}=\frac{c}{\sqrt{101\times59}\ \mathrm{mas\,day^{-1}}}=2.243\ \mathrm{kpc}.}
$$
The fitted angular decelerations describe subsequent evolution; they do not relax the bound already imposed at launch. They are observer-time phenomenological coefficients, not directly identical physical decelerations in the source frame.
The <stellar parallax> distance $2.96\pm0.33\ \mathrm{kpc}$ is compatible with the constant-<velocity> bound $3.435\ \mathrm{kpc}$, but its central value exceeds the decelerating-launch bound by $0.717\ \mathrm{kpc}$, or about $2.17$ quoted standard deviations if the uncertainty is treated as symmetric Gaussian. Even its one-standard-deviation lower endpoint, $2.63\ \mathrm{kpc}$, exceeds $2.243\ \mathrm{kpc}$. At $D=2.96\ \mathrm{kpc}$ the respective inferred intrinsic <speeds> are $\beta=0.885$ and $\beta=1.300$. The latter is a concrete violation of <causality> under the symmetric model assumptions.
Thus the supplied distance favours the constant-<velocity> fit. This is a consistency comparison, not a full statistical model selection: uncertainties in the fitted <proper motions>, their correlations, intrinsic asymmetry and the distance likelihood would be required for that.
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