While I agree with you for the most part, I have to at least chime in on Shannon's ex-husband. I'm not an expert on first pregnancies, the Canadian school system, or how difficult it is to find a job in Canada, but:
After Shannon moved away, she had to finish high school, find a job, and get to know Trevor first. I don't remember the exact time Shannon moved away, but if it was around 8-9 months before Becca was born, it would have been a very narrow timeframe for Trevor to be Becca's father.
Which I personally think is more likely:
Shannon gets pregnant with Becca and is sent off by her parents. She graduates high school and after a while starts her job at a burger joint, where she meets Trevor. The two get to know each other, start dating, Trevor gets Shannon pregnant, shotgun wedding and a few months later Lilly is born.