Hi Julia,
If I understand this thread correctly, the facts (I've put questions where the info doesn't seem to be on the thread) you seem to have are:
*1841 Worfield, Shropshire: Robert MCLELLAND, 20 (remember, ages were rounded down in 1841), tailor, born Scotland
*1842 Bridgnorth, Shropshire: Robert MCLELLAND (age, occupation?) - father John MCLELLAND, currier - married Mary LAYE.
*1848 Wellington, Shropshire: Robert LELLAND (age, occupation?) - father John LELLAND, victualler - married Maria NOCK.
*1851 Madely, Shropshire: Robert MCLLEAND(?), 30, tailor, born Wigtown Scotland, is with wife Maria, son Robert aged 6 b Bridgnorth (presumably a son he had with his first wife - there is an appropriate birth, Dec qtr, 1844), children John 2 and Betsey 4m both born Madeley (presumably children with Maria) and 2 step-children from Maria's first marriage.
*1861 (place?): Robert MC LELLAND, 42, tailor, born Wigtown Scotland, is with wife Maria, children Robert 16, John 11, Elizabeth 10, William 8, Rebecca 6 and a step-son.
*1871?
*1881 Madely, Shropshire: Robert MCLELLAND, 62, born Scotland, tailor, married (nobody with him),
I don't find the change of occupation of Robert's father - currier in 1842, victualler in 1848 - a concern. Remember, this was a time when horses were the normal means of transport. So, when one stopped at an Inn, the first concern was to look after the horses (riding horses or carriage horses). John may well have looked after horses at an Inn in 1842, then taken over running the Inn by 1848. The old occupation "ostler" (someone who looks after horses at an inn) derives from "hostler" (the host of an inn or hostelry - who possibly did both jobs if it were a small inn).
The question is, can you work back from the known to find out more about Robert's origins in Wigtownshire, Scotland? I think it would be very unsafe to do so unless you can find more information to help pinpoint exactly where he was born and the name of his mother. His father's name and occupation(s) are known from Robert's marriage certificates but nothing more. He doesn't seem to have followed Scottish naming traditions for his children (otherwise his first son would have been named John). But if he did and that was just one glitch (given that the second son is named John), one would expect the first daughter (Elizabeth) to be the name of Maria's mother, the next son (William) to be the name of Maria's father (which we know it is from the marriage certificate), and the second daughter (Rebecca) to be the name of Robert's mother (unless, of course, his mother was Elizabeth which had already been used ...).
Pre-1855 (when statutory registration began in Scotland) many many children were not baptized, or were baptized in churches the records of which have not survived and/or have not been indexed. So pre-1855 records in the IGI and on ScotlandsPeople are a very incomplete record. There may well be no record to be found of Robert's birth/baptism.
As Robert was already in Shropshire by 1841, his parents might also have moved to England or might have died - one can't assume that they will be in the 1841 census in either England or Scotland. But, if they are, I'd be looking for an occupation like currier or victualler for John.
Some other possible - but much more difficult - lines of enquiry which might reveal further clues could be whether there was a death notice for Robert in a local newspaper, or a gravestone with a monumental inscriptions, or a will ...
Best of luck,
JAP
