NO, the specified E field is not implemented as a displacement current. It is simply the E fields according to Maxwell's equations.
As for "then when is injected backwards the phase of the fields should change so that the Poynting vector is now negative (towards negative values of z). ", we can discuss more:
The source will propagate along the direction as it is specified. Internally it suppresses the propagation opposite to what you define (if it has no suppression it will propagate along both directions). For example, when you specify this source is propagating along -z, it supresses the propagation along +z. This is why the grey region in the source is designed for. If your imported fields are designed to propagate along +z, but you specify it to propagate along -z, please check that the H field phase is negative.E field does not change phase.
One more side note: when you use transmission to get the relative power, the monitor surface normal is always possitive , but the Poynting vector is positive when along + axis and negative when along - axis. Thus you get + or - transmission. In physics it is always positive. You can extract monitor surface normal:
?getdata("monitor","surface_normal"); it can be 1,2,3 for xyz, but all positive.
I hope this clarifies.