I have same problem, power on magsafe but no display usb power. I solved my problem by replacing logic board.
Perhaps your power board is broken? replace the power board and see.
I am tying to fix mine with no screen, no usb but there is power to my macpro via magsafe.
I have replaced power board but same NO SCREEN.
Does anyone know how to test the 27” screen it self? I dont have fancy LCD screen tester