What lore is there for Ustalav regarding witches specifically? I'm going to be playing a Varisian witch in PFS soon, and I'm torn between choosing Varisia or Ustalav as her nation of origin. Is there anything interesting I should know?
The Carrion Crown Player's Guide offers a little bit of insight into the status of witches in Ustalav.
Reportedly, despite rampant prejudice, witches are at least somewhat respected as rural healers and spiritual leaders, particularly in isolated communities where there's little alternative. Additionally, Ustalavic witches are often associated with the mysterious ancient Kellid pantheon and ritual sites scattered throughout the region--although whether a given witch's patron is actually associated with the eldritch forces varies by witch.
Changelings are also noted in the guide for being appropriately interesting player choice for the adventure, and the Haunting of Harrowstone (Carrion Crown 1 of 6) has an article about changelings which mentions their association with witchcraft in passing.
The Briarstone Witch comes up in the Strange Aeons AP which starts off in Ustalav.