As for the "small universe" charge, I think that overlooks the sheer breadth of the story I'm telling. If all these characters were on the same ship participating in a single adventure over the course of a few days or weeks, then you could validly argue that it was guilty of small-universe syndrome. But the whole purpose of the book was to give an overview of the whole large universe that the Federation encompasses and engages with. The novel features three ships having largely separate adventures, each of which could be its own self-contained work, as well as showcasing various political, military, and diplomatic leaders; and it spans eight months of story time.
Well yeah, in universe, as I said, you can justify it. It was just too much for me personally.
As for Mayweather deserving a first officer's post, the Romulan War novels did establish that he served on a number of different ships in the war, and clearly gained a lot of experience, even if those books didn't have room to highlight it. And yet Travis himself acknowledged in the book that he wasn't sure he'd earned the post. So I did at least lampshade the issue.
What I remember from the Romulan Wars books (average that they were) was that Mayweather got bitchy with Archer and then spent the books on various ships that seemed to all blow up. As an aside, did those books reconcile Archer and Mayweather? If they did, fine, if they didnt then that animosity gets handwaved away very quickly in this book.
Its not just Mayweather, Reed should never have been made captain at this point either (he wasnt ready or experienced for it), but I felt that was done much better and was ok with it at the end.