I think you are correct. Erik (Eirik Jakobsen) Heng would be the father of your grandfather Jacob Heng. He had at least one (possibly up to 4) cousins also named Jacob for the same grandfather. (Confusion among the various Eiriks, Jakobs, Arnes is easy... at least not QUITE as bad as a long long series Ole Olsens...)
I was looking at the various pieces of information at familysearch.org and that lists a WWI draft registration form for Jacob Alfred Heng -- that threw me into confusing him with Jacob A Heng in various local censuses who WAS a son of Arne Heng (brother of Eirik).