By trade
you can't =( *sorry*
No
Yes.
By dying
YOU CAN'T, WITHOUT BEATING THEM!!!!!!!!!!!!!!!!!! or being beaten
hacks. and pokemon that are able to evolve with the dusk stone won't until you get the new pokedex (after beating the elite four)
yes
There is a fire stone in the solaceon ruins.
Kyogre and regirock-regice-registeel
You get it as your starter Pokemon or use action replay
Sky pillar which only appears after defeating the elite four.
The game does open after beating the Elite Four. You simply select Continue, and you will be back at your house after you have beaten the Elite Four and Champion. :3