Wouldn't I have to just place a diode in the direction like diode B? that would isolate the motor from any connections other than the solid state switch wouldn't it?
No, it must be in the opposite direction to #diode B because it on the ground wire not the supply wire.
If it were fitted incorrectly
(as per your amended diagram) it would allow current from the battery
+ to flow through the controller and then return back to the battery
- through the diode, alarm and DC-DC converter.
If you then tried to operate the throttle, any high motor current would instantly melt the thin black ground wire that would effectively be completing the circuit.
It would also prevent the light from working when the bike was powered up if it wasn't grounded through the Alarm connection.
Or wait, do I even need to connect the horn negative from the motor? cause the light will have negative connected at all times from the alarm unit right?
Provided the ground connection from the alarm is a permanent ground
(not just switched to ground under alarm conditions) then the black wire going to the control harness
(and the third additional diode) would not need to be fitted.
Alan