Is profit earned from mediating contracts with contractors and managing work under an assumed name permissible or impermissible, given that the employer will not pay for the effort expended if asked to do so?

What you did by adding a commission for yourself to the costs, in a deceptive and fraudulent manner, is Islamically forbidden, as it is consuming people's wealth unjustly. It is a betrayal of trust and forbidden deception, unless you clarified the matter to the employer and they agreed to it.