Dimming Cabin Lights for Take and and Landing during hours of reduced light is to allow for the pax to become acclimatised in the event of an evacuation.
Leaving Nav lights on, usually signifies activity on the aircraft, engineers, cleaners etc, indicates to emergency services and the tower, ground crews there may be people on board.
In aircraft where no flight deck camera is installed, the SOPs can include the requirement for cabin staff to remain in the flight deck while crew take comfort breaks. Other reasons have been highlighted, other procedures will not be discussed on these forums for security reasons.
There is no difinative answers to your questions due differing SOPs, however, the dimming of cabin lights is standard practice.