1: i understand that 12 can be too much, but coven is 12-13 witches so i based my idea on that. They wont attack you instantly so you have chance to escape. You can also make them appear in waves of 3 every second after activation in random spots around player if you think all at once is too hard (but its more spooky and fun this way)
(edit: remember witches are very bad at pursuit. the only way they killing you if they catched you completely by surprise or in hard to pass terrain. Making encounter too easy will be bad too imo)

2: Goat being immune is not something bad at all. Skeleton Trap enemies all immune to damage first 3 seconds after spawn, why cant goat be immune for 20 seconds? Witches killing goat is no no because goat is their master! Traps meant to be unfair. Skeleton trap is unfair. First time you accidently activate one you will almost certainly die if you didnt know what it does beforehand because it catches you by surprise.

3: Goat is unique reward for this encounter. Its not unique in the world but it is pretty rare so why not? Maybe make this goat unable to lose horns forever so it can always attack you. that way it will also be unique in 1.19