I was just wondering, if you get Maps to load directions and start navigating, then turn off the data access, can Maps function fully, or does it need to constantly pull data to run directions?
Using offline map data, you can definitely run navigation without a data connection, either mobile or WiFi. I verified with Maps 9.31.3.
As far as India is concerned, if I am to use my Google Maps with Internet connection and start the turn-by-turn voice navigation (in Andorid 6.0.1 Maps version 9.31.3) I can turn off the connection to Internet and still the navigation will continue to guide with the loaded route. However if the path that you takes deviates from the route the app will recalculate the route and will change the instructions. This recalculation will not work.
If I had downloaded an offline area, then not only even this recalculation will work I even do not need the Internet connection on the first place.
This offline usability will vary from country to country.