Let me start by letting you know that my wife and I are the on island resort managers of Naigani. I tend to prefer the diving we have here at Naigani...otherwise we wouldn't have taken the job here
The question of whether to split your time or not depends a lot on how much time you are going to have. If you only have a week, you may want to just stay at one place. With more than a week you could do more than one place. Well, you could easily pick two from Wananavu, Volivoli, and Naigani to do in a week as they are not too terribly far apart, and don't require a flight. But all are far enough from the Somosomo Strait that I would not advise combining any of the others with Somosomo if you only have a week.
We would love to host you, but if you have a week or less and really want to do Somosomo.....you might be better off just doing that one place. From Somosomo you would have to take a flight to Nadi and then have about a two and a half hour taxi ride (I think that is about right) to get to Volivoli or Wannanavu. Or, you would need to fly from Somosom to the Suva airport at Nasouri and then about an hour taxi ride to the Natovi Jetty where we would pick you up for a 20 to 30 minute boat ride to Naigani Island.
Here at Naigani, we are at the head of the Bligh waters. Lots of sites within five to fifteen minutes of our beautiful beach that have fantastic, healthy, soft coral and white tips on nearly every dive. We can also dive sites that the liveaboards, and only one or two land based resorts dive. Places like Wakaya, E6, and Mount Mutiny. Those sites are all about an hour away from our beach. I believe we are as close, or closer, to those sites than any other land based operation.