What are best practices for using Yoghurt as a Marinade?
To answer part 1. I recently watched the Chicken Tikka Masala episode from Blumethals "Further Adventures in Search of Perfection". The recipe calls for  marinading the chicken for quite a long time. A transcribed recipe says 10 hours: http://www.dominicsayers.com/documents/ChickenTikkaMasala.pdf.

They also go on to compare the results of a yogurt marinade and a yogurt-less marinade on chicken under an MRI scanner. I don't know if this is valid, but the yogurt apparently had an effect.