Congrats!!!
For the tickets, I would suggest to book only after she has the visa in hand because you never know what's going to happen in between. However, I booked my tickets through an agent here without making any payment and made sure the deadline to make the payment is after the interview date so I don't have to risk losing money if I need to change the dates on the itinerary. Not sure if it works that way over there.
I believe the consulate rejects visa application for a reason so if your fiancee has all her paperwork and required documents in order and answers the questions truthfully, then there should not be any problem in getting the visa!
Good luck on her interview.
Cheers.