When you change the set voltage 13.6 to 14.8 then 14.8 is now the set voltage. (charger voltage) there are no more "stages" it is a single stage at 14.8. There is no "Boost", and no Float.
Now whatever the batts are at, they will rise to 14.8 under that 75a current. When they get there, since that voltage is constant, amps will now taper. The charger will stay at 14.8 forever and now so will the batts until you unplug the charger.
On your edit, your choice to return it or not if it sometimes will not go into boost as described, but when you modify it to be a single stage 14.8 you don't care anymore about all that anyway.
Absorb voltage is the voltage Absorb is done at while amps taper. First you have to get the batts to that voltage (during "Bulk") Some chargers like the PM do constant amps at their rated max amps during Bulk to get the batts to Vabs. That is called "current limited" at max amps so it stays at 75a (if that is the rating) because it can't go higher.
