1. Yes, if the decree of divorce is granted on contest then the decree if valid and effective provided the ground of divorce is also valid in India. So all decree passed on contest may not be valid unless and until the grounds on which the USA court granted decree is also a permissible ground in India.
2. Whatever paper work you may have done the wife can always file case in India for maintenance unless and until she remarries and gainfully employed.
3. he can does on the ground I have mentioned in the first point.
4. Yes under the circumstances I first mentioned.