sAx, the MEL only refers to pre-departure. I think that was covered earlier in this thread.
The ECL/QRH is what matters after departure, ie. pushback.
For example, on the Embraer that I used to fly the APU shut itself down during push back, inbetween engine starts, due to an overspeed.
The MEL states that the APU must be secured prior to departure, CBs pulled etc..
The ECL states that the APU must be turned off, that's it.
See the subtle difference?
Not that that example is relevant to this discussion but the use of MEL and the ECL/QRH is.