Can anybody with a valid employment authorization card can work within the validity of the card even though his adjustment of status was denied and been asked to re-apply for it ?

If your EAD card obtained based on marriage to US citizen, it depends on status of your I-485. If I-485 is denied, EAD card is automatically revoked. Though USCIS won't ask you to return but you won't be able to renew when it expires.