The real reason behind the Finish is a bit absurd, I think it needed more context to understand why the other rich families would put up with it, at least for their own daughters, or you could just accept it as satire. Because there are rich and powerful families that can get away with what they like and would pit young women against each other for entertainment. They're manipulative and cruel, or at best thoughtless.

I did think the horse race just seemed like the Grand National with a lot less jumps and horses, and yeah, it is a dangerous race but it's not certain death if you fall off a horse, and that was lacking a bit of the tension the rest of the book had.