I love a mystery as well! I know...Where is Isaac in 1870...1880...?? I'm guessing that he was missed by the census taker or did he travel out of the county/state for work? I think i searched a bit for him in other locations but don't think I came up with anything. I was assuming that Isaac and Rebecca married before 1867 but why was she enumerated as Alford and not Allen?
In answer to a previous question you had about who Rebecca's father was...Arthur or Luther. I'm pretty sure, based on census records, that Arthur was her father.
My son-in-law, Jim Allen, is descended from Mary Jane through her son Homer William Allen. How are you connected?