The source of the 6025A at 12 kV is the transformer which is connected to the grid. For a fault in the station, none of this current flows through the earth. Since it doesn't flow through the earth, it doesn't contribute to GPR, step-voltage, or touch-voltage, and does not need to be considered.
I'm not sure what you are getting at by considering one 69 kV tower away from the station as a central station. The worst case fault will normally be a SLG fault on the 69 kV bus in the substation. The substation will be the central station. The substation at the end of the 69 kV line will be a terminal. The current of importance is the "Total Earth Current" at the central station (your substation). This is what causes GPR, step-voltage, and touch-voltage at the substation.
I don't know why you want to "find which tower directs the maximum current into ground." FCDIST does calculate the current into earth at each tower, but not all of this flows in the substation grid. At towers close to the central station, the earth current flows from the shield wire into earth and ultimately to the terminal substation. At towers close to the terminal station, the earth current flows from earth into the shield wire to get to the terminal station ground grid. Unless you are worried about potentials around the towers, tower earth currents do not need to be considered.
I suggest that you go into CDEGS Help and look at the section "FCDIST: An Overview".