Interesting letter. I am one of those you mention with a FTM site. You bring up some problems.
I have John Gerrard marrying a Jane (last name unknown). Perhaps she is Jane Newton? I only show one child, Elizabeth Gerrard who married Benedict Calvert. Perhaps John and Jane had another daughter Jane Gerrard?
I have Gerrard Hutt married to Anne Jackson with a daughter Jane Hutt who married William McKenney. Was Jane Hutt a daughter of Gerrard? She is not mentioned in his will, while all the other children are mentioned. This omission has always made me doubt the inclusion of Jane as a daughter. But I left her in because previous researchers had included her.
So perhaps an earlier researcher allowed this Jane to be slipped into the wrong family, which could be easy to do with all the commingling going on at this time. And I added to the problem by not checking the previous works.
I would be interested to see your charts for the McKenney and allied families for this period. If John Gerrard did marry Jane Newton this will add another confusing link into this family tree.