as to the level-off segment
By reading FCOMs, I assume the usual procedure would be to use managed mode for both vertical and lateral guidance. The point FINAL mode usually engages is when the aircraft passes the final approach fix after which only one stable descent path is defined by a flight path angle set at around -3 degrees. I am not sure what this "level-off segment of the vertical flight profile" means. Is this level off segement is the one that follows the final approach fix before the missed approach point?