My ancestor Elizabeth TOLAN/TOLLAN has marriage banns registered in 1853 to a Charles DEVINE in St John's Port-Glasgow. However no official marriage documents appear to have been submitted.
As statutory civil registration did not start until 1 January 1855, the record of the banns is likely to be the only record that exists.
On Charles' birth certificate in 1867, what date is given for the marriage?
It's very common for a widow to be listed in a census under her maiden surname. Sometimes even married women appear with their maiden surname. This is because legally a woman in Scotland does not lose her maiden surname when she marries. In legal documents she will be named as xxx yyy or zzz, where xxx is her given name(s), yyy is her own (maiden) surname and zzz is her husband's surname. (She will never be listed as xxx yyy zzz - always as yyy
or zzz.)