Neutron beta-decay electron endpoints ($E_{e,\max }=(m_n^2+m_e^2-m_p^2)c^2/(2m_n)$)

In the neutron rest frame with a massless antineutrino, the smallest electron total energy is $m_e{c}^2$. At the upper endpoint the neutrino becomes soft and the electron and proton recoil with opposite equal momenta. The proton is not at rest there.