Dehydrated means death of thirst, i.e. the opposite. Typically caused by getting stuck in a tree after having fled from birds, but can also be caused by an overseer who locks the dorf in somewhere, as well as being hospitalized with no water source (but that's VERY obvious, because of the fortress halting cancellation spam).
Alcohol poisoning is generally manifested by a dorf dead in the tavern in a pool of his vomit (which is what caused the suffocation death), often mixed with spilled drinks. Susceptible dorfs will typically be served drinks and go unconscious but recover a number of times before the accumulated lung damage does them in, but the "helpful" tavern keeper/performer frequently rushes up to the dorf who just managed to get upright with a new goblet of booze to keep him down. Due to an oddly delayed reporting of death (where dorfs frequently are reported "found dead" only after some apparently blind and/or mute dorf has hauled the corpse to the corpse stockpile) it can sometimes be a bit tricky to see the connection. Also note that death in the tavern can come from booze fueled lethal "non lethal" fist fights.
Purple needs are not interruptible. They're generally caused by a long pent up accumulation of need (such as a number of years without a temple). Satisfying that takes a long time, but when done a repeat is generally kept at bay with shorter green activities.