Life In 19x19

Actually in Go every points in board is resonable except line1.

When you play in 5-5 ir show you are not concentrate about the conner to much cause your opponent can stay alive in 4-4,3-3. On the other hand, 5-5 show that you are interesting in moyo and big point in middle of the board, by the way ...

My friend told me that the difference between Kyu and Dan is:
Kyu and some new Dan players ability to play are not stable. It means sometime you play as a lower kyu cause Kyu usualle play by sense not reason. So don't be scare if you thing some time you move is very silly.(Me too LoL)

So first of ...

If you play in a Joseki and your opponent didn't play in the move it could be;
Easily you may play in that point cause it doesn't be joseki anymore. Next,you have to use your skill to read the valueable move.

Is black in the down-left conner can be killed?