Is it permissible for a Muslim man to prevent his wife from removing her hijab, especially after she had previously agreed to it, knowing that she believes she was coerced into it, and is this considered sufficient grounds for her to seek a separation?

You have done well by adhering to righteousness and guiding your family towards goodness. The hijab is a religious obligation incumbent upon your wife, as is prayer. Her excuses, such as your forcing her, or your family's openness, or that she might wear the hijab but not pray, are worse than the sin itself. Be patient with her and remind her of the texts that encourage modesty and warn against ostentatious display. If the intention behind the hijab is covering the face, the matter is easier due to the existence of scholarly disagreement. It is not permissible for your wife to seek divorce without a valid reason, for the Prophet, peace and blessings be upon him, said: "Any woman who asks her husband for a divorce without any harm (to her) will be forbidden the scent of Paradise." You are not obligated to divorce her unless you wish to do so.