1) Use lags to affix to the tree. Use deck screws to put your decking on.

2)Go with treated. It will save time, and after buying paint and stuff, it will probably save you money.

3)For the lags, yes, pre-drill. For screwing your decking down, no.

4)Depending on your span, and choice of decking, joists could be a good idea. You can get away with using some 5/4 board if you have some joists.

5)As mentioned above, if you use joists, you can get away with 5/4 board.