You can not instruct for night flying.
This means that as you already said, if a PPL student's flight lands after night begins, that part of the flight during night time was not an instructional flight and the student was a passenger. Being a passenger flight you can not be paid and the passenger can not log the flight time to count towards any licence or rating.
With that in mind I expect that you will not plan to remain airbourne after night time and fly for free (pay for the aircraft?) but you would only do it when an unplanned circumstance caused you to be airbourne after night begins..........but I bet if it became a regular thing your RTF or FTO would be a little miffed at you freloading with their aircraft time and fuel.
As for people with night qualifications but not the curency - if they are IR holders then they can legally carry you as a passenger. You can not be paid but you could offer some advice which can quite legally be ignored by the pilot.
For people with just the basic night qualification and no IR - if they have not done the required take-off and landing - you can not be a passenger and your licence says you can not be an instructor. What you can however do is pay for or share the costs of the aircraft and act as pilot in command while your passenger "manipulates the controls" in order to gain the required take-off and landing(s).
As an instructor you should have no problems with a well briefed passenger flying the aircraft........but you may not be happy having to pay for the privelage.
Remember that flying actual IFR from the right seat is not as easy as you think and trying to scan cross cockpit without some practice can cause disorientation.
Regards,
DFC