Some thoughts on Becca=Biogirl theory:
1) Timing: Becca was born on Mar 15 23YA. That means Shannons missed period was June 8 24YA if you assume a 280 day period, or June 15 24YA if you use the more popularly known 9 month period. In the backstory, Frank and Shannon spent the weekend "Doin it 'n doin it 'n doin it" on June 3&4 24YA. While that still might be a huge coincidence, I suspect that CNG was trying to live up to their name by telling us about a day that was more important than it seemed. (And to Quote Abby at the end of that scene, "Did you really just tell me a story about how you and Shannon spent a weekend doing it.")
1b) With the above in mind, Shannons sudden departure in late June/Early July would fit the timeline for 'girls parents find out she is pregnant, sends her to live with relative'. Her ending message of "It doesn't matter, I don't want to see you anymore" does seem like it could come from the move not being her idea, but she has to go along with it.
2) Trevor is (probably) not Beccas biodad: According to her bio, Shannon Moved away, finished school, and began working at a fast food restaurant, where she met Trevor. If it happened strictly in that order, Becca would have been at least a year old when Shannon and Trevor met. Even if the timeline gets compressed (Ie Shannon was working during her senior year), Becca would have to be unreasonably premature to be Trevors. From a narrative standpoint, this makes Trevor a reflection on Frank; both fell in love with a girl who had a previous child, Frank loved the child like his own, Trevor didn't.
2b) However, nobody ever refers to Trevor as Beccas stepfather, or adopted father. Socially at least, Trevor is recognized as Beccas father.
3) Becca probably knows this. If she truly is the Machiavellian mastermind that she is always made out to be, and is as enamored with Frank as she appears to be, she has to have done the math at least once. Personally, my own theory is that she did a paternity test when she was 18, and the results either way set into motion her 5 year plan.
3b) If the above is true, it does re-frame some of her lines throughout the VN. There are a couple times during the first pool night, (Frank: "But you're my step daugh..." Becca: "Except I'm not...I mean only for a couple more days"). Even moreso, when Frank is negotiating with Becca about her plans and the subject of Abby comes up, Becca usually responds with something along the lines of 'I know she is the most important one to you'
4) Side stories: We already had Lillys side story, about she figured out she could have daddy issues with Frank. Abbys side story is coming up, and it looks like it will be about her dealing with the fact that Frank is not her father. CNG has said that Becca will also get a side story, which has left a few people on these forums to wonder why Becca needs one seeing as she is apparently happy to be a part of the daughter-harem. One possibility that would give Becca a side story with similar emotional scope as Lillys and possibly Abbys would be if it came out that Frank was Beccas biofather.