Could glutamate excitotoxicity really occur after abruptly withdrawing from a benzodiazepine and, if so, is there any way to be able to tell that it occurred, such as a seizure indicating excess glutamate? would a seizure be necessary as a precursor?

Excitotoxicity: To quote the Wikipedia article about it, "In evolved, complex adaptive systems such as biologic life it must be understood that mechanisms are rarely, if ever, simplistically direct." Nowhere is this more true than in the brain, which is the most complex adaptive system we know of. Instead of worrying about this, I'd focus on the practical clinical challenges of BZD discontinuation.