Get the Indian permit(s) well in advance for your schedule and advise your pax that last-minute changes (last-min I mean anything within 3-5 business days before the flight) may or may not be possible with the Indian DGCA.
Depending on who you use for the permits the time getting and / or revising them can vary considerably.
A good estimate is 5 business days for Indian landing permits and 3 for overflight - don't count on it though.... bureaucracy deluxe!!!!
Have a safe trip!