The Allies didn't want Belgium as a speed bump, the Allies thought the Germans were going to attack through Belgium, and offered to set up defensive lines on the Belgium-Germany border, this, Belgium turned down.
It was Germany that only attacked Belgium as a diversionary strike, and attacked through Luxembourg, and the Ardennes into French and British held lines, the weakest part of the line though as the British were already moving north to counter the attack on Belgium.
Neither the BEF, France or Belgium were used in Plan D as a stalling component, its just thats the way the Germans played it. And Belgium never even felt the full force of the Blitzkrieg. I've already explained it all before.