Yes, it does look like Naomi was a twin, the child was Mary d. 3 Apr 1856.aged 7 m 25d. It put's her born ~ 9 Aug 1855 and Naomi claimed the 8th of Aug as her birthday.
I don't know who Elijah's parents were. I had tried to narrow it down to one or two of the four men who might have been possible parents, never came up with an answer. I also tried to thing the names of their children, but, they are named mainly for the Hitchcock ancestors. The son James Isaac, well his name does not fit into the Hitchcock names.
Also, have never found Elijah on the 1850 census, have you?