Hi Richard,
Just been doing a little digging with the SMI & IGI & I know the following is pretty flimsy but...
It's possible the two Mary Verrall's are cousins. Mary cr. 1770 d/o samuel & Mary & Mary Verrall d/o John & Ann cr. 1767. I tried to find evidence that Samuel & John were brothers but failed. I found that Samuel is very likely the s/o Cephas & Mary Verrall nee Mockford married 1734 Ringmer, he was from Rottingdean & Mary from Ringmer but didn't find evidence of John as a son but we all know the IGI is not complete!
The children of Edward Crunden & Mary Verrall include a son, Samuel....possibly named after Mary's father?
Of the marriage of the other Mary Verrall to William Henty, I can find no children.
Noticed that the entries for the Verrall's & Crunden children are all LDS member submitted, so beware.
In conclusion, I would explore both Mary Verrall's & keep everything open until such time as some information comes along that proves or disproves a theory

Good luck,
Jane