1. The ground is required to be seen as on the ground of adultery no marriage is annulled here. So if the same is used as a ground for nullity in USA it is not valid in india ,more so, when it was passed ex parte.
2. She can file a fresh case including case of dowry harassment.
3.There is no need to contest it. If she can files any case you can not take advantage of the decree passed in USA.
4. Yes,she can file case for divorce in India and file case for maintenance also.