Relying on transcriptions, I couldn't see anything wrong with Caroline bap 1834 Kings Brompton, dau of James and Mary, marrying James Thorne and being the mother of Mary Ann and Rosa Thorne. They look like Mary's grandchildren to me. The only thing I could think of was if James Venn had married twice, to two Marys, and so this Mary who left the will was a step-mother, not bio mother, but still the relationship in the will is wrong, so I don't see that there's much of a case for that.
Maria is more interesting...
Like you, I couldn't find a Maria Lyon to fit. However, I found an Elizabeth Lyon on the 1911 census. She is aged 69, says she is a widow and was born Somerset, Kings Brompton, so everything correct except her first name!
https://www.ancestry.co.uk/interacti...nSearchResults
Her son on the above census was Joseph Lyon aged 35 b Deeping St James, Lincolnshire. His birth reg should be in Bourn District, but (after I'd eliminated a wrong Lyon reg in 1873) I found this:
VENN, JOSEPH VENN LYON
mmn -
GRO Reference: 1877 M Quarter in BOURN Volume 07A Page 396
In 1881 this Joseph Venn and his mother, Elizabeth (still not Maria!) are living with widower, John Hill, and Elizabeth is his housekeeper (place of birth not known). Joseph and Elizabeth are listed as Vernon on Ancestry, but looking at the page (Joseph's entry is on the following page) I can see why!
https://www.ancestry.co.uk/interacti...3197_3202-0639
On the 1891 census Joseph (now) Lyon is a boarder in the same household he was in before, but Elizabeth isn't there, just widower John Hill, one of his daughters and a grandson.
https://www.ancestry.co.uk/interacti...618.1572732679
When Joseph married in 1899 (as Lyon) his father's details have a line drawn through.
If you have FMP it's here:
https://search.findmypast.co.uk/reco...2F00246026%2F1
I can't find a baptism for Joseph (only looked for Venn), nor Elizabeth in 1891 or 1901. (I looked at women born +/-5 of 1842, born Somerset living in Lincolnshire, with any name, looking for someone who had some part of their name that I recognised, but didn't find anyone).
Given her age, surname and birthplace in 1911 I feel it's very likely Joseph's mother is Maria Venn and that she called herself Elizabeth for some reason. I can't find any marriage for Venn to Lyon with any first name, but I guess whatever she called herself, she didn't ever marry Mr Lyon.