Hoping to resurrect discussion on this thread

If you scroll back on this thread you will see I am a descendant of Holmes Cornwell, via James Cornwell and Hannah Greygoose. Since my initial posts on here I have done a lot more research, collaborated with other descendants and built my tree considerably. I've DNA tested and also tested my paternal great uncle who is the oldest living relative I have on this line. Happily, our results verified much of my paper tree and explained a few question marks I had over some ancestors.
I'm now tracing backwards from Holmes Cornwell and Elizabeth Day. I am happy that William Cornwell and Elizabeth Holmes are the parents of Holmes Cornwell. I see that it is generally agreed that William Cornwell and Sarah Blacklock are the parents of William. Why? A "most likely" scenario? Is there more evidence than the very patchy Fulbourn registers? I can find no will for William or Sarah so has anyone else uncovered one? I'm transcribing the documents relating to the legal spat they had with Sarah's family with the help of Redroger, but so far it doesn't seem to shed any light.
I also question the William Cornwell / Ann Arber marriage. This is often said to be the son of William Cornwell and Elizabeth Holmes, but I cannot see why this conclusion has been drawn?! What am I missing? Why has the William Cornwell / Sarah Wallis marriage in 1795 at St Andrew the Less been discounted? I can see 4 children attributed to a William and Sarah Cornwell baptised at St Mary the Less Cambridge, but the first baptism is 10 years after the marriage at St Andrew the Less, which seems an awfully long time for it to be the same couple... However, there isn't any other marriage that is leaping out as more likely! The 1840 burial for William Cornwell aged 73 at St Mary the Less says his abode is Potton, but I can find nothing in Potton relating to him or any of the children baptised at St Mary the Less. (Tbf, the Beds FHS CDs aren't a patch on the Cambs ones and my Potton one doesn't play very nicely in my laptop!)
So I've come on here to see if anyone who has trodden this ground before can explain or is willing to share their research, or if they have gone down the DNA route, pool results!

I know there are loose ends in my Cornwell research, they were such an extensive family with a complicating way of intermarrying through the generations.
For anyone interested, I have gone back a number of generations on Elizabeth Day's paternal line, although I am stuck on Mary Lemon's line (I have put a note on Ancestry about all the Swaffham Prior baptisms being listed on there as occurring in Sutton which has given rise to the Mary Northrop scenario). I have also gone back a number of generations on Elizabeth Holmes' line and it is all on a private tree on Ancestry I am happy to invite any Cornwell descendants to view!
Any thoughts more than welcome!
Karen