Are we allowed to omit 在 in 在…里，在…内 structure?
In my experience(correct me if there's any exception), the structure '在...里'(as well as '在...前', '在...后', '在...上', '在...下' etc), when followed by '的' and then a noun, serving as an adj, is allowed to omit 在.
Examples: 房子里的人，公园里的树，信里的字.
And when it's after a verb as an adv, you can't omit 在.
Examples: 站在房子里，种在公园里，写在信里.
btw, I don't think the first sentence is correctly parsed:

Two thirds of Californians live within 15 miles of the coast.
三分之二的加利福尼亚人居住在离海岸15英里以内的地区。

15英里以内 -&gt; no more than 15 miles
离海岸15英里以内 -&gt; no more than 15 miles from the coast
离海岸15英里以内的地区 -&gt; areas that are no more than 15 miles from the coast
居住在离海岸15英里以内的地区 -&gt; live in areas that are no more than 15 miles from the coast
there's no '在...内' structure in this sentence at all.