Guided mode with optical flow and lidar without "terrain following"

I am flying my quadcopter using set_position_target_local_ned commands in guided mode.
I am using optical flow+lidar and no GPS.
Currently the quadcopter exhibits terrain following (the commanded vertical velocity, vz = 0).
I have two questions:

  1. If I send non zero vertical velocity and the terrain changes, what will happen?
  2. Is it possible to fly without terrain following in my configuration? How? That is, I want the altitude to be determined by the barometer and use the lidar for the optical flow base navigation solution.