It's like this. If you want to go through the wall on the pic, you would slide north.
Why ? Look at the map, you are facing north, and you want to go straight forward. That means you go north.
In this case, you would want to slide east to go through the wall.
You are facing east, the wall is infront of you and press "6" and slide through it.
Nope that wouldn't work. Other people (and mobs) can't see you in the air before you jump. That means that you could be hanging 200 feet in the air and everyone would still see you as standing on the ground, until you jump.