I see you’ve been very busy! That’s very interesting information, thank you.
After reading your message I returned to the 1850 Henry County IL Federal census page where I found Arnold, Emily, and John Congdon (and family), along with Matthew Buck and quickly came across the Stackhouses.
I’m going to check those census references you gave and look for another family of interest, the Cratsenberg family. Benjamin’s father-in-law was Andries Cratsenberg (or variations of Cratzinberger). Andries was living with Benjamin & Sybil in 1860 in Pepin County WI. The Cratsenbergs were generally of New York, but we believe Benjamin & Sybil were possibly married in Pennsylvania around 1840.
Disappointing is my complete failure to find gr-grandfather Benjamin anywhere in the 1850 census, when he should have been in Illinois and probably Henry County. I’ve found him under various spellings of his name in the 1860 US, 1865 MN, 1870 & 1880 federal census pages.
I don’t believe I have ever communicated directly with your father-in-law, however my initial source after learning the names of Benjamin’s parents, for the Congdon Family was Earl Perry Crandall’s “New England Families” http://worldconnect.genealogy.rootsweb.com/cgi-bin/igm.cgi?op=GET&db=newengland&id=I21397http://worldconnect.genealogy.rootsweb.com/cgi-bin/igm.cgi?op=GET&db=newengland&id=I21397 and in his sources he lists the following attribution: “Notes of Lawrence Congdon, Owego, NY; dated 29 Sep 1998”. All I have currently on Amos Congdon comes from that source, and indirectly the work of Lawrence Congdon.