I also believe that Water Elemental could use it's free action move after an attack to move into a Steep Hill.
I doubt that this new way of moving is covered at the moment.
Since the water elemental is not
taking a move action, like a creature getting pushed is not
taking a move action then i would assume that the water elemental actually can enter steep hill zone and / or go move through wall of thorns after it pushes a creature.
The limitations / requirements from Supression orb, WoT and Steep Hill are targeting creatures taking the action of moving - which the Water Elemental isn't doing as part of the attack sequence.