1. No special treatment necessary unless you require a different finish, although quality decking oil does enhance the longevity and aesthetic of your timber.
2. This is only really necessary for timbers with a high tannin content like Merbau. Post installation treatment would be the best choice as you can sand down the deck easier that hasn’t been pre-oiled.
3. Timber is best, softwood treated and certified C16 minimum or C24 is best unless you can afford hardwood.
4. KD would suit best in your environment, AD would contract soon after installation.
hope this helps, feedback from other installers would also be of assistance here.