Are metabolic causes of dementia preventable?

Yes. As a rough estimate about 5% of dementias are treatable/preventable, so screening for things like vitamin deficiencies and untreated hypothyroidism should be done as part of an evaluation of patients with early memory loss.
Yes, but . Are you asking about treatment or prevention & are you thinking of dementia or delirium. There are relatively few metabolic causes of dementia (b6 or B12 deficiency, electrolyte imbalance, anoxia) and they all begin as delirium, subsequently becoming dementia. Delirium can be treated, preventing dementia.