The Glasgow Post Office directory of 1840-1841 list 3 William Moffats
- a surgeon, at 158 George Street
- a last and boot-tree maker at 108 Stockwell Street
- a grocer and spirit dealer at 656 Gallowgate
The second one can be discounted because he is easy to find in the 1841 census, aged 40, so too young to be your William Moffat.
The surgeon was aged 55 in the 1841, so born 1791-1796, so he too is also too young to be yours.
The grocer and spirit dealer was listed at 1004 Gallowgate in 1837-1838, at 656 Gallowgate in 1838-1839, and from 1839-1840 onwards the listing is
Mrs William Moffat.
Formal etiquette would describe a married woman as Mrs followed by her husband's name. If she was a widow it would describe her as Mrs xxx yyyy where xxxx is her given name and yyyy is her husband's surname. However there is no guarantee that the compilers of the directory stuck to strict etiquette. The point being that the change from William Moffat to Mrs William Moffat does not necessarily mean that William Moffat had died. He might have retired from the business, perhaps owing to ill health, leaving his wife to run it.
If the business address had been in Robertson Street, it would have been reasonable to suppose that it was your William and Christian's business, but it's not, so the conclusion I arrive at is that the Glasgow Post Office directories are not going to help you. Nor will the valuation rolls, because at that time they don't name all the small householders.
So all this has been a rather unhelpful red herring, I suppose.
You might like to have a look at the directories for yourself. They're online at
http://digital.nls.uk/directories/browse/pageturner.cfm?id=91047488