= Solution
Take inertial axes parallel, with frame $S'$ moving at $+v\widehat{\mathbf x}$ relative to $S$ and origins coincident at zero time. The <Lorentz factor> is $\gamma_v=(1-v^2/c^2)^{-1/2}$; $\Delta x,\Delta y,\Delta t$ are separations of the same pair of events in $S$, and primed quantities are their separations in $S'$. The transverse coordinates are unchanged. For two events on an inertial clock's worldline,
$$
c^2\Delta\tau^2=c^2\Delta t^2-|\Delta\mathbf r|^2.
$$
The <proper time> $\Delta\tau$ is that clock's elapsed time; uniform <speed> $w$ gives $\Delta t=\gamma_w\Delta\tau$. Coordinate intervals between spatially separated events must instead be Lorentz-transformed with their spatial separation included.
For the round trip in the station frame, the rendezvous distance is $ut_{\rm out}$, and the return has the same <speed> and distance. Thus $t_{\rm round}=2t_{\rm out}=10a/(\sqrt7c)$. Its departure and arrival both occur at the station, so $\Delta x=0$ for these endpoints:
$$
\boxed{\Delta t_{S,\rm round}=\frac{10}{\sqrt7}\,\mathrm{days}=3.780\,\mathrm{days},\qquad
\Delta t_{B,\rm round}=\gamma_v\Delta t_{S,\rm round}
=\frac{25}{2\sqrt7}\,\mathrm{days}=4.725\,\mathrm{days}.}
$$
This uses the return <speed> as measured by the station. The elapsed time aboard A, apart from an ideal instantaneous turn, would be $\Delta t_{S,\rm round}/\gamma_u$.
For the crash, A's station-frame <velocity> components are $(3c/5,\sqrt7c/5)$, while B's are $(3c/5,0)$. Here $\gamma_u=5/3$ and $\gamma_v=5/4$. <Four-momentum conservation>, with no separately escaping radiation, gives the wreckage totals in the station frame:
$$
\boxed{\mathbf P_S=mc\left(\frac74\widehat{\mathbf x}+\frac{\sqrt7}{3}\widehat{\mathbf y}\right),\qquad
E_S=\frac{35}{12}mc^2.}
$$
In B's original <inertial frame> the same totals are
$$
\boxed{\mathbf P_B=\frac{\sqrt7}{3}mc\,\widehat{\mathbf y},\qquad E_B=\frac73mc^2.}
$$
The invariant wreckage <mass> is $m\sqrt{14/3}$, exceeding $2m$ because <internal energy> remains in the wreckage. Its <momentum> cannot be specified without naming a frame; both natural frames have been given.
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