Okay, a re-read of Wanda's testimony suggests to me that Judy was actually summoned after the fall of Goodminton, because it was stated that Haffaton experienced a long period of growth and was only stopped by Easteros and Westeregg. Haffaton was definitely growing when it took Goodminton. So the timeline allows for it to be Delphi, if Turning and Croaking are indeed indistinguishable to a Ruler's Thinkamancy. The second fact however is probably something we'll never know.
Problem. Olive Branch was Haffaton's Chief Caster before Goodminton fell. And she didn't join Haffaton until some time after Judy Gale was summoned (remember that her turning presumably involved the Arkendish, which Judy Gale helped Charlie acquire). It probably wouldn't have been long
before Goodminton fell, given how much smaller Haffaton was at the time, but it was still before.
Besides, Haffaton was specifically mentioned to have a Predictamancer already, Maya Calendar.