Putting the car in 'N' when stopped for any length of time is OK, It is something that I always do.
You can select 'N' while on the move but why would you want to?
You can shift from 'D' to 'S' or 'S' to 'D' while on the move, if you couldn't you would not be able to make the best use of the 'S' facility.
If you don't stay in 'D' while in bumper to bumper traffic how will you make progress?
The purpose of the 'S' mode is to enhance the performance. I use it a lot, for example when pulling out into traffic at a junction; when overtaking; when joining busy roundabouts etc. Don't be afraid to let the engine reach the higher rev range, 6000rpm, you will not damage it. You will surprise the drivers of other cars at the improved acceleration.
Vic.