It pushes off of the bottom platform, the cube accelerates by pushing off of itself (if it didn’t it would flatten into a pancake). And because there’s a downward force on the bottom of the cube it pushes down on the platform

If momentum is conserved then neither A or B is correct, the box should be an infinity thin sheet. If the portal is half way through the box if it maintains it’s thickness the box has to be moving at the speed of the portal out of it.

Licking lead once is completely fine, just don’t make a habit of it

Keep it simple, stupid