Thanks so much for affirming what I'd been thinking in the back of my head... straight to Miyajima it is! We'll save a couple of hours the next morning for Hiroshima as suggested, then move on to Nagasaki.
The dinner in Nagasaki isn't special per se; it's our first night at the ryokan which we'll be spending the better part of a week at. I've only opted for a single dinner at each ryokan because it just gets too heavy and repetetive otherwise. We couldn't possibly eat multi-course kaiseki dinners every single night nor be stuck to such a rigid schedule when we want to be out sightseeing and enjoying local restaurants as wll. As such, I've asked to only have dinner at each ryokan on the night of arrival. That way, we'll likely be tired from travel and just want to stay in and relax, enjoy the onsen, have dinner in our room, etc. At this point in our trip, we'll have had countless kaiseki meals at countless ryokans and so I'm not expecting anything special from this particular one. I have some sense of regional cuisine in Nagasaki, but mostly of the Chinese influence so I really don't know what to expect from our ryokan dinner. I have purposefully chosen to eat at restaurants that do not require reservations for the rest of our meals in Nagasaki as we'll have had our fill of fine dining in Tokyo, Kyoto, and Osaka.
I guess we'll wait and see what time we leave Hiroshima. If it turns out to be late and we're worried about our arrival time, we'll call the ryokan and either let them know that we'll be arriving later than scheduled or request to have dinner the next night and just go somewhere casual upon arrival.
Anyhow, thanks again for your input and feedback; we'll definitely be taking your advice for Miyajima!