In Marlin setting the #define min_software_endstops false lets you go negative after homing.
Z_PROBE_OFFSET_FROM_EXTRUDER is for the difference in the height between the probe and hotend. If you have min_software_endstops set to true, then that setting won't work as expected.
Z_PROBE_OFFSET_FROM_EXTRUDER is for the difference in the height between the probe and hotend. If you have min_software_endstops set to true, then that setting won't work as expected.