Lots to think about!
So in 1871 Jacob wd be living at Plas yn Clawdd, Gardden, a farmer. With John 15, Elizabeth 13 & Jacob 9 while wife Mary is a few miles away in Pentrefelin, Morton Above with Louisa 15. I wonder why. (I have looked at 1833-43 map of area but can’t find Plas yn Clawdd or Pentrefelin. However, there’s a Pentre y clawdd just east of Gardden Lodge. Can't find them on the current OS map which you sent me the link to. Are you familiar with either of these places?)
1871 mistake re Mary's age: 10 years out. Could this be an enumerator's mistake? The other puzzle is Louisa's birthplace : Ireland or Staffs? Maybe the mistake is in fact in 1861 census & she was born in Staffs like several other of the children. I guess I need to look at census returns for her as married woman to see which birthplace she gives. But then are the marriage records going to be straightforward? I'll pursue this later. There's a similar 'mistake' made in relation to John in 1881 who is recorded as born Rhosllanercrugog but in 1861 born Staffordshire. Were mistakes / confusions like this fairly common?
re Blackwell Index/Mary Wright: what is the Blackwell Index? Can I access this? many of my family were Methodists & it sounds a plausible source for Mary (though gnu has pointed out this is a common name). Would it have marriage & christenings too for nonconformists? I haven’t yet got a record for Jacob & Mary who were married by the time of 1851 census. [Their son Jacob was married at Cannock Register office 1883 so was Methodist by then.]
I'll try following up Albert & Louisa.
Thanks again for all your help.