The low voltage side is usually the output side. So putting the secondary windings closer to the core will insure the maximum amount of flux passes through the secondary windings.
Not sure what you are asking. Transformers are wound for the best magnetic coupling possible to keep loses to a minimum.
The low voltage winding is placed closer to the core because the high voltage winding requires more space due to its MV insulation. If designers put the LV winding outside the MV winding, the tank would be larger because the _MV_ winding size would be about the same size. More insulating oil would be required, and the leakage reactance would be significantly higher. Remember, higher leakage reactance means that the voltage regulation (more aptly voltage drop) under load will be higher, which is not desirable. And, of course, a larger tank and more oil make for a more expensive transformer. Edit: MV winding is about the same size for either configuration, due mainly to insulation requirements.
