It is some time since I looked at this family, so I will have to go back and look at my notes.
I do remember I could never find James or his wife in the 1841 Census.
My guess is both he and his wife Helen Black died before the census.
There is a Helen Wood buried 1838 in Paisley, but it's hard to know if this is her.
I can't find any baptism for her in Whitburn or surrounding areas.
Or a baptism for James Wood.
Looking at my notes, they were married 1807 in Whitburn, so possibly born about 1880/1890.
They must have moved to Paisley between 1814 and 1820. Daughter Margaret was born in 1814 in Whitburn, and daughter Jane in 1820 in Paisley. Have you found any others between these two?
I have Mary 1807, (baptised less than a months after they were married), Janet 1809, Margaret 1814, Jane 1820, William about 1822, and Helen about 1828. Margaret, William and Helen's baptisms not found. Helen married Michael Cochran 1850 in Paisley. She died in 1892, parents given as James Wood 'farmer', and Helen Wood, maiden name Black.
Mary born in 1807, married John Mclntyre and died in 1889 in Paisley (John McIntyre was already dead). Her parents given as James Wood, 'cowfeeder', and Helen Wood maiden name Black. The informant of her death is Michael Cochran "Brother in law".
William born about 1822, died in 1905 in Dunbarton. Parents given as James Wood, 'cowfeeder' and Helen Wood maiden name 'Blackwood', which is interesting. William's son, another James Wood, was the informant. It's not difficult to imagine he was unsure of her name. Both of these grandparents would have been dead many, many years before he was born. It's also entirely possibly she used both Black, and Wood, as names from time to time, as married women did in Scotland. Maybe she ran them both together and became 'Blackwood'?
What is the information about her parents on your Janet's death certificate?
What makes you think James was in the military?
I haven't come across anything to suggest this.
The baptisms in Whitburn of Mary in 1807 and Janet of 1809, give him 'of Wester Reeves' - the initial query on this post. Plus the death certificates of three of his children give him as a 'farmer' or 'cowfeeder'.
Most likely a small farm near Whitburn.
It's probably a better idea to continue this on email rather than write reams here.
If you'd like to send me your email address on a Private Message, I'll tell you about some other stuff I have found.