Like we told you before, I honestly do not think there will be an issue with the CO. You included the child on your eDV application, documents proving your relationship with the child (such as B/C and passport) are in your name. Now of course there is the issue of the immunization card which is in the birth mother's name. I expect the nurse in charge of registration to question why the last name on the immunization card is different from that on your 2NL and the child's passport, and that can easily be explained by you telling the truth - which is the child is adopted hence the immunization card was issued with the birth mother's name. I don't expect the nurse or the doctor to request to see adoption papers. You just need to check and cross check that the medical records that will be sent back to the embassy is listed with your last name, and one of the ways you can do that is to confirm the the child's form is filled with your last name.
I see no reason why the CO would request for a DNA test! Not unless you and the rest of your family are used to treating the child different from the way you treat your biological kids. One of the things they do at the interview is to quietly observe how family members are relating to one another while waiting to be called up to meet the CO. The people that often fail this observation test are those with some kind of arranged marriages or those trying to claim a child that was not previously listed, and if the CO suspects a couple with a child is behaving in an odd way, then they request for a DNA. I see no reason why a DNA test would be requested in your case.