The M+ and M- on the speed controller are wired to the brushes, polarity does not matter (this is what powers the rotor) The Power + and power - go to the battery been charged. Its probably worth fitting a 20 amp fuse just in case someone hooks up the batteries around the wrong wa...
On the topic carbon monoxide, it’s slightly lighter than air by specific gravity (search ‘engineering toolbox for specific gravity for gases’ to get a detailed chart). The numbers are so close that arguably it will just mix with ambient air instead of rising to the ceiling. The big iss...