If the enemy wants to move toward a hero that is already on a node with 2 others, does it stop movement as if it has hit a wall / obstacle or as if there is no node to travel to?
Page 10 of the rules state
Only three models can share a node at the same time, regardless of how many are friendly or enemy models. If there are already three models on a node and another model moves onto that node, the players must push one of the three models already on the node
So I suppose the enemy does move and then the players select which model gets pushed, however this doesn't answer the question of whether the players can elect to have an enemy be pushed. Strictly speaking it must be an option but isn't stated that if possible a character should be pushed. Enemies don't typically push each other but if there is a situation with 4 enemies all wanting to inhabit the same space, it breaks the 3 model rule. A reasonable reading is probably that the enemy pushes the character if possible and otherwise an enemy, but I haven't found any ruling yet.