Shooter landing on a player who flopped

[Nevadaref]

A defender is allowed to take any spot on the floor provided that he gets there legally, which means without using illegal contact to reach the spot and arrives before any time or distance restrictions.

For guarding an airborne opponent, the defender must have obtained his spot on the floor before the opponent became airborne.

Now we can analyze your situations. If the defender falls to the floor prior to the shooter jumping, the defender has a legal spot, and it is not a defensive foul when the shooter subsequently lands on him. It could be an offensive fouls though.
If the defender falls backwards after the shooter is airborne, he has not met the requirement to be in his spot on the floor before the opponent went airborne. Therefore, it is a blocking foul if the shooter lands on this defender.