yes, all pieces fit but like you said dev won't tell ever (unless moves to subs and makes uncensored version).
I mean we had that argument before and everything has been analysed and said, but as they say, not by everybody.
In my opinion this is unlikely for a meta reason as well, namely that this game - or its story - make a point out of being what it seems like being. Our daughters not only are claimed to be above 18 as in some other games, they need to be for the story to work. They are not "adopted" because of skipping patreon or other rules, but for the story to work. Granted, a hidden "bio dad" wouldn't kill that at all because the story runs on their official relationship, however, I would consider it more likely that a story element was introduced which forbids Becky to be Frank's son, than that a surprise (or not-so-surprise to those who theoreticize differently) come out later on that she actually is his bio daughter.
Though my guess as well as yours is that it won't be addressed at all. And to reiterate something I said before: I still think that CNG never paid as much attention to the exact timeline as some here do (which is not meant as a criticism of the speculation whatsoever, collecting clues and speculation is fun in fictional stories).