Yes you can trade from past Pokemon games...I think only heart gold/soul silver though...
after you beat the Pokemon league
You have to transfer from other games so no
Pokemon Black and White are not compatible for trading with any of the other previous games.
No. Pokemon Black and White are the only current games with no legendaries not belonging to their region. It can be traded from other games, though.
Yes you can but you can't trade from black to pearl or white to diamond because the games are not compatible with each other because the Pokemon in black and white will come up as ? in the pokedex's of the other games. But you just can't trade to pearl from black or white sorry. However you can trade between black and white using infrared connection. (the other games don't have infrared that's why the game cancels the trade between pearl and black or diamond to black).
There are no pokemon after pokemon blACK and white but there will be more pokemon games in the future.
No. Generation V Pokemon are only available in generation V games.
yes it can.
If you are looking for Zorua or Zoroark, they can only be obtained in Pokemon Black and White, and they can not be transferred to other Pokemon Games.
You cannot trade Pokemon on from other games (unless it's Pokemon black/white) until you have defeated the elite four.
You can trade the DS Pokemon games onto Pokemon Black and White when you have the national pokedex and have found sort of the pal park in black and white.
You can't, the game won't let you transfer items from any other game apart from Pokemon Black or another copy of White.