A long answer to your short question.
My wife and I stayed at Matangi Island 4 years ago in April for 11 days. I have not kept up with cyclones so the information I provide for reefs is as it was 4 years ago.
This is a smaller and higher end resort (in terms of cost) and is the only one on the island. It is in the Taveuni area. This would put you close to Somosomo straits area. In the Straits, we did dive the White wall, Rainbow passage and another which I havent looked up in my computer dive log. This was the area where we found the blue ribbon eels. My favorite dives in this area however, are Yellow wall and Noel's wall (with The Edge in between) near Matangi. White wall is cool but Yellow and Noels wall were fantastic in my opinion - more wall, more color, tons of healthy plate coral on top, fish life everywhere and of course yellow and purple soft coral. Most other diving was around Qamea and Matangi. These dives were where we saw white tip reef sharks, huge jellyfish, octopus and all manner of south pacific creatures including very large clams (though I did not see one seasnake). All the diving was drift diving (i.e. no moored dives).
The resort had one dive boat (large AL and holds 6-8 divers plus crew). Somosomo straits diving is not easily reachable from land based locations like Viti Levu and possibly not Vanua Levu depending on where you stay. I dove almost everyday with most being two tank dives and if there were sufficient divers, a three tank trip to Somosomo straits with lunch included on a beach (extra $). Shore diving and night diving can be arranged.
My wife and I brought all our own gear - checked through from Denver to Nadi. Then a puddle jumper from Nadi to Taveuni and then boat pickup to Matangi. A very long day and a half. Take note of the puddle jumper weight limits and charges. They did not charge us as the plane was about a third full arriving and leaving. Having just paid $130 US for overweight charges in Exuma (100% full plane), be prepared if it happens.
All bures are essentially oceanfront and separate from each other. We did not use the tree house bures but did tour them. The only complaint I have is that the doorways are low (I am 67). I think the max number of guests based on number of rooms is about 22 to 26. There was a point in our stay where there were 6 guests total and the staff but did have up to about 16 guests maximum at another point as I recall. I would say about 30% were divers but at one point it was 40-50%.
Night life was pretty much non-existent and there were no children. The roosters did wake us up like clockwork (as roosters do). There was no TV but they did have phone and internet service in the office. This is a quiet resort.
When we were leaving they were constructing a new building I think for restaurant and some other function but it doesnt seem that it made it their website (old pics?). Check out the website as it has many photos of the island, rooms, dive boat and more information than I can provide here. The owners were very good about answering email questions.
As for your question about a wedding, it would depend on how many are in your party and who is paying. This could be a very expensive wedding.
You might also consider resorts on Taveuni which I believe are less expensive but maybe larger. It would provide access to Somosomo straits diving and maybe trips to Qamea and Matangi (Yellow and Noels wall). I heard rumors of a resort on Qamea that was being opened.
One thing to do if in the Taveuni area is to visit the Tavoro falls (aka Bouma falls) on Taveuni in Bouma National Heritage Park. They dont call Taveuni the Garden Island for nothing. There are three falls with the first being very easy to get to. The two higher falls involve a bit of a hike up but worth it if youre up to it. If you plan to jump off the ledge at the lower falls make sure to watch the locals jump as there are some submerged rocks off to the sides. It is easier to climb up to the ledge barefoot (slippery).
April can be a bit wet but not bad for the Taveuni area. Rainfall in the area is about half that of January but double that of July. Not sure how that works out for your wedding.