I did not find any other Arnolds living close by during that time frame. I did find some Arnold marriages in Texas County, but they were in the 1880s, after your family of Arnolds had left for Illinois. I ran through a few census records, looking for this family and in all of them except the 1880 in Illinois, his birthplace is given as "Missouri". And after looking at the actual census record, Ellen Wood IS clearly written Wood and not Moad or Mood. I did run across a marriage record index for William Arnold and Barbara Ann Mode. They were married in Missouri, but it didn't give a date, however William was living in Clinton County, Missouri in 1850.
Hope this helps!!