Duke (Trinity) does have a requirement for math. However this can be completed by taking a Comp Sci class or even Stats.
http://trinity.duke.edu/undergraduate/academic-policies/curriculum#areas
Listen, 18 on math is not the end of the world. Just need to convince them that you have strengths in other areas.
The legacy pull is pretty strong at Duke. Statistically your chances are approx 50% higher than compared to non legacy students. Dean Guttentag revealed publicly that the acceptance rate for legacy kids is approx 33%.
I suggest to apply ED and at the same time work on raising your ACT score. Even if you get deferred, a higher score in the RD round may be enough to tip you over the edge. If you can get your math score into the mid 20s your composite will be 30+.
But if you wait until RD there are literally only about 800 spots left. And 35,000+ people applying for those. Being legacy doesn’t help much in RD.