The child custody case filed by them with visitation rights is not maintainable.
If there was any right towards the child from their family it vested only with your deceased husband and not on his parents.
Since your husband died before the dissolution of marriage, now you are not a divorcee but his widow, hence you have rights in his property and your minor son also have equal share in your deceased husband's properties.
You can strongly protest their petition seeking custody and visitation rights in the court, the court will give a verdict in your favor since their claim is not maintainable.
2. Since the divorce did not happen, the conditions therein cannot be enforceable, moreover they are just grandparents, while the biological mother of the child is alive, they can be termed as others, hence others cannot have a claim as a right in this regard.
3. This money belongs to yo and to yor minor son also, your father in law cannot cheat the money belonging to the minor child.
You may issue a legal notice and file a money recovery suit agaisnt him
4. You lodge a police complaint and with the help of police you search their house property and retrieve your certificates.