Fade In on object

Hello everyone

first, please excuse my english…

There is my problem.

I have a scene where objects must appear while off screen.
It’s a little travelling on a table, then the camera turns and bang, new objects are here.

My forst action was to change objects layers so they appear.
It works but as they are quite big, and i’m in Ambiant Occlusion, the luminosity of the scene changes suddenly.

So my second idea was to Animate the Alpha of the materials, to make a smooth appearing.
But when doing this, the object which is 1st plan makes a “hole” in the scene !

A little example is better than my bad english i guess :
The file is here

You can see that the red cube is masking the rest of the scene.

What am i doing wrong ?
I guess i’m animating the wrong channel…

Or is there any way to make this change of luminosity more smooth when objects are back in the scene ?

Thanks for your help