No need to assume anything, the first two links tell you exactly what the record is.
Merchant Navy ?, yes and no, strictly speaking no, because the Merchant Navy wasn't given that title until after WW1, before that it would have been called the British Merchant service, but for practical purposes it amounts to the same thing.
The individual record isn't dated, what you see is what you get, the only available information about it is that it was compiled between 1845 -1854, so factor that into your Royal Navy service record, and you can answer your own question about whether or not it predates or postdates his Royal Navy service, and as he apparently enlisted in the R.N. in 1860 his merchant seaman record presumably does predate his R.N. service.
An unusual name combination like that, from such a small place, I think it is reasonable to assume that the R.N. sailor and the merchant sailor in Britain are very likely to be the same person., and you might be able to find some Scottish baptismal records for him, and there are other McClelland names listed in Whithorn in the 1851 census.
http://info.dumgal.gov.uk/HistoricalIndexes/Home/DisplayQuic...http://www.genuki.org.uk/big/sct/WIG/Whithorn/You might find something more about him on his ship in Australia on this site.
http://mariners.records.nsw.gov.au/shipdate_2.htm