Is the money the questioner received from social security by irregular means, knowing that she was married, unlawful, and must she return it, especially since she asked a mosque imam and he issued a fatwa that she does not have to return it because she is among those eligible for zakat?

You must repent and seek forgiveness for concealing the return of the husband after receiving social security funds, because it is deceit and fraud. As for the money that was taken: if it was from zakat and the husband was in hardship, with his salary not being sufficient for his needs, then it is not obligatory to return it if it was within the bounds of sufficiency. However, if the husband's salary was sufficient, then the money must be returned to the responsible authority, or paid to those deserving of it from the categories of zakat if that is not possible. If the social security funds were not from zakat, then the money must be returned to the responsible authority unless one's liability is discharged. If one does not currently possess the funds, they can be returned whenever one is able, even in installments.