Knot-free (clear) timber is generally more expensive than knotty. It is personal preference as to whether or not you use a clear or knotty grade of timber for your deck.
Yes you can in some cases, but we recommend that use a vapour barrier that allows water to pass through it such as weedmat. But we strongly recommeded that allow minimum appropriate height 300mm off the ground if possible. this allow the deck to 'breathe' and will mitigate mould and mildew issues that occur as a result of humidity build up under the deck.
Concrete, like all other material, will slightly change in volume when it dries out. In typical concrete this change amount to about 500 millionths. Translated into dimentions-this is about 4mm in 3m. That is why we put expansion joint/cuts in our new concrete pavements/floors, is to allow the concrete to crack in a neat and staight line at the joints when the volume of the concrete change due to shrinkage.