OP, you're overthinking it.
Here's what we know: Asuma is Hiruzen's son. Konohamaru is Asuma's nephew and Hiruzen's grandson. They all share the same last name (Sarutobi).
So if Asuma is NOT Konohamaru's father, it simply means that Hiruzen has a 2nd son that has never been introduced, and that 2nd son is Konohamaru's father. Just because Asuma is the only son of Hiruzen that actually appears in the show, it does not mean that he is Hiruzen's only son. It's never mentioned in the anime or the manga.