I have been spending a while looking for Briget's death. Can't see anything so far that looks promising in terms of following up 
She may have taken up with another man and name registrations might get more confusing. Hugh left her in the late 1860s? Mother's maiden names are being added to the search indexes. Still not 100% though. There is also the possibility as always that a mistake could have been made or it was not known at the time of reporting a death.
Monica
I am 90% sure Hugh went after the kids were born, I am related, along with my other cousins to Catherine's GG grand children and also to Hughs American kids. That presumes they have the right guy on their trees.
I do wonder about another man or if people died in a "poor house" if its possible they slipped through a decent death registration. I know there were a few bad ones of old women at the end of the 1800's in Glasgow. Ones conditions were described conditions a stepping stone between Glasgow and hell!