This is not the only case. For instance, the tops of the towers at heights 11, 17, 23 are collinear (both height differences are 6, both pairs are 2 primes apart).
Even if it turns out not to be relevant to the solution, the question should specify what happens in such cases.
Yes, exactly so.
There is another small ambiguity here. The towers 2, 3, and 4 have colinear tops. But this is the only case and not important for the solution.
This is not the only case. For instance, the tops of the towers at heights 11, 17, 23 are collinear (both height differences are 6, both pairs are 2 primes apart).
Even if it turns out not to be relevant to the solution, the question should specify what happens in such cases.
Yes. You are right, I thought I might be wrong about this.
Okay. If they are collinear, then they are visible.
As you say, there indeed many examples, even of three literally consecutive primes: https://en.wikipedia.org/wiki/Balanced_prime