1. upgrading from 215/75r15 to 215/60r17 is good option : Go for 235/60r17, don't forget to install Rogerab for better ride quality just like LUVTUV did, more ground clearance and comfort, I'm planning on it, too.
2. Variance in mileage approx. : approx 1.3% +/-.5%, IDC really
3. Should i upgrade on day of delivery for better buyback price of tyre, stock rims ... OR should upgrade after driving 100/200 kms to ensure any manufacturing defect if any : Do it as soon as you can as the used tyres and alloy wheels would increase the exchange price. Make sure to get good quality alloy wheels as you do not want to compromise on that, get all terrain tyres for better traction(MRF Wanderer)