z-min homing and z-probing normally do not play nice. You can not have z min trigger when correcting z at z=0, so this normally gives errors already with normal printers. If z min then use z probe for homing as well.
Since I would use the z-homing for leveling the x-axis, I could use z-max endstops indeed. As I don't want to do this every time I start a print, I was wondering if there is a gcode to change homing direction of z-axis and the revert it back after calibration. A dirty hack I read is to use M92 Z-1590 to invert homing dir. Any better suggestions?
It is not designed to be inverted after compilation. Not sure why the trick works. It would move in opposite direction, yes, but would not check the right endstop I'd think.
You have offsets per extruder, but these are to define position of extruders agains first extruder. So with that you need no offsets in print.
So why do you need it? Knowing what it is fore allows better thinking about what is possible.
These values are not meant to be changed during printing. You can change all eeprom values with M206 but you should not do this constantly as write cycles for eeprom are limited also they are quite high.