I can offer some speculation, and it looks like you have really done your homework, so I hesitate to do even that.
I took a look at the 1850 census (which you have listed in your sources): (https://www.familysearch.org/ark:/61903/3:1:S3HT-DTBS-JBD?i=16&cc=1401638
I noticed that it appears that 70-year old Eunice is living in a household without any other adults, with a 2-year-old Alexander and a 4-month-old Arilla Miller. I assume that they must be grandchildren, or possibly even great-grandchildren to Eunice.
On the next page of the census (and presumably immediately next door) is 41-year-old John Miller (born in Mass.), with (possibly a wife) 24-year-old Mary M. Miller, and two Miller children, one, a 7-year old named Decodatus Miller, which sounds a lot like the Deodatus in your family tree. (Looking at the photocopy of the census page, the name could have been spelled either way).
John is a possible son of Eunice. I also spotted, at the bottom of same census page (with John Miller on it), a 4-year old Simon Miller, living in the household of John Mallon, age 40 (born in Ireland) and 31-year old Ann Mallon. I wonder if Ann Mallon could be a married "Miller" daughter, who is caring for 4-year-old Simon Miller.
Anyway, I just thought I'd mention these clues in case they are helpful in some way.