So we have ma - the upwards force
No we don't. There's no "a". Acceleration comes from net force, so if the net force is zero there's no acceleration.
You need to describe things in terms of forces, not accelerations.
And there's no homogenous "stiffness of the structure". Most of the stiffness is provided by the columns, which were essentially bypassed in the collapse (at the top tllted), they were not "in the way", and hence largely irrelevant to the speed of the collapse front.
This domino model on the other hand has the main support structure in the way all the way down, and is not really falling in any macro sense (dominos are largely just toppling, and then falling off).