Does glutamate rebound, which is hypothesized to occur during benzodiazepine or alcohol withdrawal excitotoxic/neurotoxic, i.e. are neurons killed during this process?

It's possible: Alcohol is an NMDA receptor antagonist and benzodiazepines are GABA receptor modulators. Alcohol inhibits the effects of glutamate and Benzos enhance the effect of GABA; both meds cause inhibition of brain activity. When you stop either class of meds, you can experience glutamate rebound which leads to withdrawal symptoms such as seizures. Theoretically, this can cause neuron death.