Of course refusing the blackmail is the obvious choice for Sophia. She protects Dylan either way. And luckily, your "what if" scenario is irrelevant. L&P might have just offered up some other solution for those refusing the blackmail. Fact is, Sophia knows Carl well enough to trust him and her sister. Alyssa even offered up a solution herself that Sophia refuses.
Refusing the blackmail isn't the obvious choice for the player, which is really my point. Like many of the decisions in-game, there really isn't a proper reason for Sophia to choose the Filthy option. You think I really believe Sophia would think it reasonable to make the decisions I make in my playthrough? I do it because I like boobs and naked women, and the lesbian route has a lot of it. That Patricia is hilarious also factors into it.
You said the blackmail was weak, almost like if it was not a big deal for a mom to make a choice which could lead to her son being beaten if she doesn't find another solution, which she didn't had yet when making this choice (screenshot 1). No, it was not as obvious than you seem to say it, for Sophia, for a mom, being cornered about the well being of her son.
Fact is, Sophia knows Carl well enough to trust him and her sister. Alyssa even offered up a solution herself that Sophia refuses.
See the screenshot 2, Sophia is saying the
exact opposite of your statement, how she doesn't know well enough Carl to ask for help financially.
She only
considered to ask help to Carl way later after refusing the blackmail (when they went at the beach) and she was reluctant to ask for help. And she only wanted some contacts and lawyers (screenshot 3). The real solution was far to be obvious and the problem didn't seem as easy to solve than what you're implying. Asking to others to solve your problems is not the obvious solution for most people.
And about the Alyssa proposal, it was never an option, it was something impulsive (from a angry mother) and could lead to way worst, of course Sophia discarded it. See the screenshot 4.