Actually, the back is the same as one knee in that situation (the two-feet part of the equation only applies if he hadn't been touched on the way down). If the receiver maintained possession of the ball through the contact with the ground, he would be down by contact and thus unable to make a subsequent "football move".
Yes, one knee or the back is the same as two feet. Even if the receiver got two feet down, however, it would still be an incomplete pass (in the middle of the field - since getting two feet down to stay in bounds in considered a "football move").
The player is not down by contact until he makes a football move. This exact situation occurred in the Falcons-Bears game this year. Until a receiver makes a football move, it is the same as juggling the ball.