The biologically active form of vitamin b1 (thiamine) is thiamine pyrophosphate which acts as a coenzyme in several biological pathways. Less thiamine means less ATP production because thiamine's role in dehydrogenase-catalyzed reactions involved in producing energy, so neurons become more susceptible to damage/impaired cellular function. Chronic alcoholism leads to thiamine deficiency because of impaired absorption or inadequate dietary intake (so you'd see other deficiencies that you mentioned as well) and the clinical picture is described as "beriberi", which comes in two forms and is probably why it's emphasized so much because it's a very classic presentation also as you mentioned. Hope this helps!