Allowing an extra day before your international is a good idea, though inter-island flights have improved dramatically. Chathams Pacific, who I believe are your current sole option, were fairly reliable, the airlines before were awful. If your international flight is in the evening and you leave the islands in the morning, then it allows some 'things going wrong' time. If you are working on a tight schedule- don't. Nothing happens on a tight schedule in Tonga, which is one of its charms. At least make sure you have trip insurance.
Mala is lovely and does have good snorkelling; I have visited the island but not stayed there. I stayed at Lucky's
Tonga Accommodation Vava'u - Lucky's Beach Houses - Vava'u, Tonga twice and highly recommend it; it is also a bit out of town and is joined by bridge to Vava'u. You can self-cater there, and Lynn's cooking is fantastic; exploring the local villages was great too. Taxis back from town are inexpensive, and Kurt was good about dropping me in. The snorkelling there is ok. Sometimes the dive boat could have trouble getting to me for pick ups at low tide, which was the one down side; I'd either have to wade out to them or they would meet me at another resort. After the dives, I'd usually go into town to do any shopping I needed and get lunch- the Aquarium cafe does great food. It's a popular hangout with sailors working their way through the Pacific, so you might be able to do some crewing.
I also stayed at the Adventure Backpackers in town. This was clean and nice enough. Being in town, you are near what passes for night life on Vava'u, and can hire bikes or walk to explore the island. Vava'u itself doesn't have much in the way of beaches, so you'll need to get off the island for snorkelling or sunbathing. There are a few other hotels on Vava'u, which were out of my price range (but I am a very budget traveller). We picked up some people staying out at the Blue Lagoon, and they had great feedback about the place- the island was certainly beautiful, however it is not cheap!
Here
Where the whales sing - Vava'u, Tonga Travel Blog is one of my blog pages for Vava'u- with pictures, in case you are interested. Hope you have a great time, which ever chain you choose!