If the top of the central column extended out to the edge of the stairs the outside of the column wouldn't appear in the matte at all, as he would be thrown down it and not the space beside it.

Vader throws Palpatine from the the space you describe, the death pit is right next to the lift.

As I said the matte is the problem and it's not a good matte so I haven't a problem with it being replaced.

In any case Palpatine is falling down the inside of the tower not the centre of the station.