Remember, at least between christmas and the beginning of January, the south of Mozambique up to Vilanculos is crowded with southafricans!!! And they bring everything with them, that allows them to make terrible noises - bakkies, motorboats, quads...
I wouldn't worry about the weather. Ok, there is a possibility of storms during this time, but these occur locally. The rain is not too bad, most of the days it's fine and hot.
There is a public transport in Mozambique on the main routes. If you have to leave them ( i.e. to get to the coastline / beach ), you have to organize your own transport. That means, you have to ask the local drivers / truckdrivers, to give you a lift. If this is the way you like to travel, so it is "possible", if you don't like it and you only want to use the busses and minitaxies, you might have a problem to get along somewhen...
But I heard of many people, who travelled by hiking and looked for their accomodation, when the day started to end. They always found a pension or small lodge, but be aware: The standart in Mozambique is ZERO (and the further north you get, the more "zero" it becomes !!!
If you travel in Mozambique with an own/rented car, take a 4x4, because nearly all campsites at the coast are situated within the dunes along the beach and it is impossible to get there only with 2x4.
fengi