Is it forbidden for a man to walk holding the hand of his niqabi wife in public places, knowing that some justify this by saying it indicates that the woman is married?

A man placing his hand in his wife's hand depends on the custom of the country. If people consider it unseemly, then it should be abandoned; otherwise, there is no harm. However, a woman wearing a headscarf that does not cover her hair is a display of adornment (tabarruj) which is impermissible. The husband also incurs sin for this, as it is considered Qawadah (pimping/allowing indecency).