How can merging two sorted arrays of N items require at least 2N - 1 comparisons in every case?
What is meant by "lower bound" in this case is a lower bound on the worst-case number of comparisons. In this case, it happens to also be an upper bound.


  The lower bound has to be something that is true for every possible N and every possible combination of elements in both arrays, right? 2*N* - 1 seems more like it would be the upper bound since there is no way there can be more comparisons than that.


No, it just needs to be true for some input (for infinitely many N).

For example, when we say that $\Omega(n \log n)$ is a lower bound for sorting, this does not mean that every array requires $\Omega(n\log n)$ comparisons. It just means that some arrays require that many comparisons, and thus there is no $o(n\log n)$-time algorithm for sorting.