I am glad I stayed the night in Miyajima. It is a delightful place, and much more so after day trippers left. And I thought it an excellent place to decompress from the emotions of visiting Hiroshima. JMO.

I haven't checked, but I would think you could arrange the timing for just one night in the area: As long as you are in Hiroshima by 11 or 11:30 a.m., you should have time to visit the A-Bomb Dome, Peace Park, and Peace Museum before leaving for Miyajima. (And that's even the case if you need to get to Miyajima in time for a ryokan dinner.) The next day, you can stay on the island as late as you want before moving on -- or at least, as long as would work if you want to visit Himeji en route to Nara.

I'm not sure I would recommend staying on Miyajima if you feel you need to spend 2 nights in the area, as it isn't a particularly good base for visiting Hiroshima. Again, JMO.

>>taking the train may simply be an easier option?
yes, for a wuss :)

>>If I was to stay in Hiroshima on the night of the 15th, would you recommend still leaving there on the 17th?

Yes, but leaving from Miyajima on the 17th.

>>Would you recommend staying in Miyajima or Hiroshima?

I'm recommending both. Hiroshima first, by necessity.

The idea of going to Hiroshima on the 15th is to relieve the schedule pressure on the 16th and 17th.

Hiroshima and Miyajima can both be visited on a day trip from Kyoto. A single overnight trip is fine. Two nights is better, imo obviously,

It's a matter of what you might want to do in Kyoto on your 4th evening/night there, the 15th. You could use that time to travel so that you wake up in Hiroshima on the 16th. An issue would be what to do about dinner on the 15th.

Wouldn't getting a train over plane be less wussy? Lol.

I prefer trains.

Obviously, mrwunfrl and I disagree about how best to allocate time. Your call!

>>train over plane
Wes, I will agree with you about the relative wussiness if it makes you feel better.

The one night trip south does work. You just take a small backpack with what you need for the overnight and next day. The other stuff gets shipped to a hotel in Nara or Tokyo. At some point you stash your backpack into a coin locker at Hiroshima station. Might be able to leave it at the museum or just carry it in.

You could visit Hiroshima on either day. And still maybe squeeze in a visit to Himeji-jo.

One thing that might help in deciding when to visit Miyajima is the tide table: visit-miyajima-japan.com/en/prepare-your-stay/previsions-meteo-et-marees.html

Luggage-forwarding is called takuhaibin; here's the info:
http://www.japan-guide.com/e/e2278.html

Thank you everyone for the great advice :)