[Wassahman]

I think that you are almost correct, but there is still a chance that the moneylender will refuse to pick a pebble. There is only one sure-fire way to have her not married and her father's debt removed. Simply, as she picks the pebble, she would not show it, then trip and fall, like women of her time would "stereotypically" do, and drop the pebble. With no form of proof of what pebble she picked, she would simply say, "Well, what ever pebble is in the bag, must be the opposite of what I chose."

I think this is the best way for her to get what she wants, logically.