I've actually sneaked into the grove - where Nettie is - using the Key of the Ancients before introducing my character to the people in the grove. What I haven't tried is using invisibility to sneak past Kagha, the worried Tiefling parents, and then go interrogate Zorru. Have you tried this? Does the invisibility effect get removed if you open the door that leads out to where the Rite of Thorns is being performed?