Numerical answers

6. Minimum $T_{e^+}=937.8$ MeV in c.m. frame
Minimum $T_{e^+}=3.45$ TeV in frame with $e^-$ at rest

7. Minimum $E_\gamma=2.04$ MeV

8. $E_\gamma=339$ MeV,     $\theta=0.2$ rad