Sorry for your loss Richard. The lack of records on her parents may indicate one or both may have died shortly after her birth. I'm guessing to find any real answers you are going to have to visit Dade county and look for her birth record and adoption record, her parents marriage, divorce, and or death records and perhaps newspaper articles from that time period. If you are unable to travel there, try contacting the Dade County Genealogy Society and ask if they can do some of the research for you or suggest someone who can be your research proxy.

Some of the answers may be in court records, both common pleas and probate and occasionally some documents, especially land records and death records may be at the Recorders office. Some counties combine the Clerk and Recorder so check online at the County's website to see how to inquire about those records. These searches may involve a fee but they are going to be a lot less than traveling expenses. Genealogy Societies usually only charge for materials but consider a donation especially if they get results.

Since you now have her parents names you can start looking for them in records in South Carolina and Alabama. I only did a cursory search and found nothing but you may try joining some of the mailing lists and reaching out to other researchers as you did on this board for more answers. I have not seen or run across many southern Shirer researchers, especially on the Shirer Boards and Mailing list which I admin but you may have more luck with the McKineys/McKinneys. Neither family seemed to be in Florida for long before Margaret was born so it is likely they met and married there but previous census records may find them in their home states.

Good luck on your quest. Feel free to post any results you find here.