I would take the 1855 marriage record as the best bet for his parents' names, as James would have given those to the registrar himself. Doesn't necessarily guarantee they are accurate, especially if either or both parents died when he was a child. I also wouldn't put too much store in the fact that his parents aren't noted as being deceased at the time of his marriage - I've wasted lots of time looking for parents who were ostensibly still alive, but were, in fact not...
Sadly, the Longside parish records are incomplete for this time period, so you may well not find any record of the marriage, baptism or baptism of other siblings, if there were any. There is a marriage of a William Smith and Elizabeth Craig in Longside in 1810, which might tie up with the baptisms that Colin has flagged above - William Smith and Mary Wilson married in 1818. Conceivably could be the same William Smith, with his first wife dying. Could be worth having a look at these marriages and baptisms. There are also a number of infant deaths that could match up to some of these baptisms.
A trawl through the Kirk session records doesn't throw up any obvious illegitimate births for Smith and Craig either. And there aren't any other obvious deaths after 1855 of Smith with mms Craig. There also don't appear to be any Craig MIs in Longside.
It's a shame that the aunt, Jean Craig, appears to have died before 1855, as that strikes me as the only lead... All in all, a bit frustrating...
Ruth
