Can I zero out maximum acceleration to use machine defaults?
Can I just put a 0 in the maximum acceleration to use the firmware set values under M204?
Or, do I need to input the firmware value into PrusaSlicer & would it override the Firmware M204 values if I increased in PrusaSlicer?
RE: Can I zero out maximum acceleration to use machine defaults?
No you cant. If you set zero in those fields and have emit to gcode turned on it will set them to zero.
You should enter the values from your printer into those fields. That way if they are included in the gcode then they wont cause your printer any issues. That's the whole point of allowing you to set them.
You have the option at the top of those settings to not emit to gcode. They don't get included with that option so your printer wont be over ridden. Although its best practice to still enter them and set the dropdown to Use for time estimate even if you dont want them in the gcode. That way Prusa Slicer's print time estimates will be a lot more accurate.
Ignore= no M20x are included in the gcode and the time estimates are way off
Use for time estime = M20x are not included in the gcode but the values are used for print estimates
Emit to Gcode = vM20x values as specified are included in the gcode and used for the time estimates.