Taverns:
Will my dwarves still get a drink from a food stockpile and drink it in the dining room, just with an unhappy thought that there was no tavern? Has anyone had good ideas about defending the fort from visiting vampires/weres?
Dwarves do not need a tavern to drink, nor do they care about being in a tavern when they drink. Taverns are for socializing, poetry recitals, dancing, etc. They are not for drinking. I know, it's weird. We're humans, so we don't understand.
The only new thing about drinking is that dwarves want to use mugs now. They still
can drink straight out of a barrel like they did in previous versions, but they get slightly sad if they have to do that.
If you make a tavern, don't assign any Tavern Keepers or Performers, because that causes horrible bugs. Tavern Keepers in 0.42.x are the deadliest things in the fortress. They can easily take out an entire population.
Temples:
Dwarves will get a happy thought for going, but that's it, right?
They become unfocused if they can't worship. This affects their ability to work, at least.
Alcohol Poisoning:
This sounds frightening. All of my dwarves drink nothing but alcohol most of the time. So they're all going to die, and there's nothing I can do about it.
Just don't assign any Tavern Keepers, and you won't be poisoned.