Thanks all, questions answered in Tech Log.
QNH and QFF only differ by the lapse rate from here to the sea level. QNH uses ISA lapse rate, QFF uses actual lapse rate.
Absolute altitude is the distance from the aircraft to the ground through the local vertical. The local vertical goes through the centre of the Earth, not the centre of mass of the Earth.
That's the short answers. There's also a good link to an archive of another Q-code thread.
Safe flying
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Confident, cocky, lazy, dead.