Try as we might, with so much data it is possible to make errors and to get somewhere it seems that books are written with a reliance on data from others...it happens. I corrected some of the erroneous Morrison information given to Barth and others by a well meaning and persistent Morrison researcher.
So yes, it seems that in 1800 Gerrit was probably living with his son David, Sr. as there are 2 males on that census (1 over 44 and 1 who was 26-44 with a corresponding aged female. There are also 2 males under 10 and 1 son between 10-15 (David Durling/Durlin who married Ann Pierce).
The David Darling on the 1810 census in Chillisquaque, Northumberland, PA with a wife and 2 children under 10 is not likely to be the David Durling who married Ann Pierce.
David Durling/Durlin, Jr.(born 1790) married Ann Pierce in 1816 in NY and they did move to Fredonia...all of the genealogy follows correctly with the naming conventions used with their son, Abner Pierce Durling.
It is interesting to see a 'Stephen Darling' in Northampton, PA living by himself at age 10...wonder whose family he belongs to.
I noticed that Fayette Derlin was rector of an Episcopal church in Westmorland County, PA in 1857. The son of David Durling/Durlin and Ann Pierce was a 'reverend' and was very well known. I believe he eventually settled in Wisconsin. It seems probably that there are family ties with the Alexander Darling that you mention but it doesn't seem probably that it is the David who married Ann Pierce.
There is more work to be done on this family, isn't there. Good luck and happy hunting. You never know when the key will turn up.