Please excuse this delay in responding to your post; your findings generated some new leads I wanted to check out.
I'm not sure I agree with 99% probability of Mordecai being Jacob's dad, but it is certainly worth checking out. Based on your findings, I requested some other look-ups and did find that Mordecai left a will that had apparently been changed a few times. Sounds very interesting. Judy didn't have time to look at all of it, but she will copy any pages that relate to heirs. Hopefully, Jacob will be named.
Jacob and Mary died before death certs were required in Logan County, IL. Jacob did leave a will. No burial place has been found for them. Sad.
The Beck family is also a hang-up. Sure wish those earlier censuses were every name! No wills found for them.
Thanks for all the ideas/comments/suggestions; it always helps to brainstorm thru the walls.
I'll let you know if Mordecai turns out to be the dad. What a wonderful Christmas present that would be for my aunt!