To OP, I would recommend going to Rennell Island for the 5 days after your dive trip...Although I wanted to make it there during my short stay in the Solomon Islands, I was never able to. This is a massive uplifted coral atoll with beautiful cliffs. There is a fringing reef all along the island. Form my research, there has yet to be anyone that has taken any udnerwater images of the reefs surrounding Rennell. That was my plan originally and I wanted to then share these online but, as I mentioned, I never made it to Rennell. I got in touch with a Rennell Island-native that now lives in Australia and he drew me up a map of the island marking where the best snorkel spots were. I think your experience on Rennell would be very unique - note that the inhabitants of Rennell are cultrually Polynesians.
To Andreni, I have been to the Solomons but never PNG. I spent 10 days diving on a liveabord in the Solomons in May/Juen 2015. You can read a detailes review I posted on sb:
Trip Report Solomon Islands May/June 2015 - Solomon Island Dive Expeditions
The diving is very varied. If you expect the kind of big fish diving one would encounter in the Galapagos or Cocos, this is not for you. However, i did encounter sharks (blacktips, grey reef and whitetips on many dives. There are several sites that have large schools of pelagic fishes and there is always the chance of seeing hammerheads, silvertips - I even saw a sicklefin lemon shark. If there were something that could be considered singature for the Solomons, it would be the underwater topography: caves, swim-throughs, cuts, each doing wonders with beams of sunlight filtering through the water column - not too mention the hard coral that completely covers the substrate. There are abundant macro opportunities but I would not call Solomons a strictly macro destination - there are. As a selling point, there is one dive site off the small uninhabited Kicha Island that is otherwordly (read my review).
From my understanding, as far as macros/muck diving coupled with reef diving in PNG, Milne Bay might be the best option.
My humble thoughts