If you make National Merit Semifinalist, you want to submit whatever is needed to move to National Merit Finalist. Then, most of the scholarships are awarded by the colleges themselves, if you choose to apply to them and get admitted.
Regarding Questbridge, there are other schools besides Stanford in the list at https://www.questbridge.org/college-partners which offer civil engineering and other kinds of engineering. You may want to look more carefully to find them and consider adding them to your Questbridge application.