1. You don't.
2. Not really after 2.0 - there may be some limited support.
3. Both are about as fast - questing if you have done it before and have an optimized order. Grinding if you want to just knock it out as fast as you can in one place and not be bothered.
4. Whatever you like the most. Hunters are harder to box due to melee / dead zone issues but it is certainly possible to dual box them.