I re discovered your old post today....
www.rootschat.com/forum/index.php?topic=322522.02008...it has been a while

So the surnames we have re Willam are Ross, Smith and Bremner. Florence and William are consistent in both William's marriage and death certs. Just the surnames get mixed about.
If William was given up not long after his birth, it is not surprising that things were a bit mixed up for him.
Looking around again, I found this 1871 census entry:
John Smith 58 grocer b. Alford...Parents for them I think a William Smith and Isabel Cooper/Couper
Mary A Smith 51 sister b. Alford
Florence Bremner 20 boarder b. Edinburgh
Mary Mcgregor 19 servant
Jane Mcgillivray 43 servant
Address: North Back St Cottage, Forres Moray
Her birth is actually showing as Coloo Mooto Florence Bremner (this is from SP)

Born on 13 Aug 1850.
Florence married in 1875:
Florence Bremner and Jno. Stuart
Marriage: 14 Jul 1875 Cathcart,Renfrew
1881 entry for her post marriage in Cathcart, Renfrewshire:
John Stuart 40 shipowner b. Forres, Moray
Florence Stuart 30 b. Edinburgh
Charles Stuart 4 son b. London
Mary Am Smith 64 boarder b. Alford, Aberdeenshire...the same lady likely from the 1871 entry.
Ann Greenock 50 servant
She died just the following year
www.findagrave.com/memorial/182947558. Also,
http://libindx.moray.gov.uk/people/people_report_view.asp?REF_ID=NM176750My main thoughts on this are, could this Florence have had an illegitimate child (William) in 1872 before her 1875 marriage. We have here the two surnames of Smith and Bremner and of course, Florence.
We still need to find a birth reg as always!
Monica