As usual, not easy to advise on how a person ought to spend their time. You never know how you'll react to a place until you're there. I avoided the Yasawas on principle, but towards the end of my time in Fiji I decided to pick an island and see for myself what it's all about. I went to a travel agent in Lautoka, asked about places where tourists DID NOT go, and went there. I picked a resort on Waya island (not Octopus at the north end, but one of the cheaper 'backpacker' places near Yalobi settlement). I picked Waya mainly for the chance for some hiking. Snorkeling at south end of island wasn't all that great.
If you have to get back to Nadi anyway then, one way or another, you still have to leave Caqalai and spend a day traveling - the money and time to Nadi has to be spent anyway, so it isn't time and money lost. Just something you have to do one way or the other.
But to continue on to the Yasawas you will also have to overnight in Nadi, and take the Yasawa Flyer the next morning to whichever island you pick. So, you're looking at closer to 1 and a half days travel, and the extra cost is for the Flyer. Looking at it that way, you don't have a lot of time for a look. Is it a decision you can postpone? I can appreciate the lure of curiosity, but in terms of things to do, etc., you could just as easily spend the 3 or so days at one of your other destinations.
Also all comments about the accommodation places we have chosen are VERY welcome!
6 days in Lavena Lodge ? Do you know what to expect? Unless there has been some massive renovation, then you're going to find a basic room, a kitchen you can use but next to no food to buy. I spent 4 days there myself, but I also enjoy the relative absence of mass-tourist infrastructure, and the chance for some jungle and coast walking, just being out in nature. There appear to be other places on your list which will give you just as much chance to sit around doing nothing. Perhaps .. be open to a move on in less than 6 days?
It depends on how you want to distribute your activities: Lavena is for the hiking. Dolphin Bay is for the sea, snorkeling, diving. If you want to distribute your time 6 days to 2 over these activities, OK.
Mary's Holiday Lodge in Levuka is a very basic place in terms of rooms/facilities. Be prepared. The plus side for me was the family home environment. It depends on what you want to do, as usual. If you could manage so much time in Savusavu, I'd have thought Levuka should deserve more than 2 days (more interesting, IMO, for local life). Anyway, if you considered staying longer in Levuka, consider the Royal Hotel as an alternative - just for the old colonial atmosphere of the place - or split your time between Mary's and the Royal, which is what I did.