Ur questions can have 2 aspects
1st- U got a seat and u filled Status Retention form.
If u did so and then u can’t take part in Mop Up Round bcoz first of all it is not allowed and secondly all of ur certificates will already be taken by ur college in which u took admission.
If u try to do so then under Malpractices in Counseling u will be banned from sitting in counseling.
2nd- U got a seat but u didn’t filled the Status Retention form then u can definitely take part in Mop Up Round.