Whoa!! All the best for the interview! Order the GCMS notes right away. Thats the new system in place of CAIPS. You could order them online and get an email. Apparently, its faster than CAIPS.
I wouldnt say that Im surprised that they called an interview. Common-law is not too common down here. I believe the VO's expect to see weddings with a lot of guests and the works. But you should definitely get through without hassle. Hopefully, they will stamp the visa immediately after the interview if your luck holds
All the best. I had a comprehensive list of interview questions with me. Will dig it up for you in a min....