1. Would you say the pack is worth 50% off for mainly the world and some of the BB&CAS?
Vampire pack is about VAMPIRES, everything else is a nice addition. If you don't plan on playing vampires (Which you 100% should) - this is not for you. With that said...for 50% off I will say perhaps.
2. If I have no vampires living in any house, would there still be game generated NPC vamps?
Yes.
3. Will the CAS face details be available only to vampire sims?
Vampire CAS details are for vampires only.
4. Can garlic be used for anything but keeping vampires away?
Of course. It is a vegetable and therefore counts for "Any vegetable" requirement.
5. Can vampire stop being a vampire, and if so, will s/he keep those perks earned?
Vampires can be cured, but it is a bit bugged. Basically, first you want to make sure they are a clean slate in terms of their perks and weaknesses.
6. Do vampires live forever? If so, the family photo wall might be kinda limited as they hardly need to reproduce... Or does it vary, or can it be activated/deactivated like a perk?
Vampires are undead and therefore immortal. This is not Sims 3.
7. Are any features restricted to the Forgotten Hollow world?
Plants can be grown anywhere and in case of one specific plant - at any season. Vampires can live anywhere. Nothing stops you from having a vampire pack themed house in, say, Willow Creek.
Anything else I should know? Do you want the occult that is actually well made, that comes with downsides AND offers different play style? Vampires are for you. Do you want only items? Reconsider. There are CCs for that and some do not require vampire pack. With that said I do STRONGLY recommend it. Give vampires a chance. 100% best occult EAxis made for Sims 4.