Is it obligatory to return money taken from Zakat funds held by the mother? Is it permissible to give it as a charity from the husband's money with his knowledge? And is it obligatory to inform the mother of this?

You must repent and return the money you took unjustly. It is not permissible for you to take money from your husband without his permission. You can tell him that you need the money to repay a debt you owe. You are not obliged to tell your mother what happened. Rather, you can put the money back in its usual place and mention that it might be from charity funds.