Already been answered see post #15
In this case, I think you could cut either one, provided the MCU GPIO pin connected to the sensor's output is tristate (pinMode = INPUT), which would normally be the case when reading the sensor. But as I said in post #17, a low side switch is the simpler circuit, but not as fool-proof as a two-stage high-side switch.
No it would not but disconnecting ground could cause problems.
In this case supply. You need a P-channel mosfet for that.
The sensor and processing board draw a combined current of less than 12mA.
If you would use an Uno R3 I would suggest to simply power the sensor on/off with an Arduino pin. But since you will be using a racehorse (the R4) instead of a workhorse (the R3), that might not be wise. Good luck finding a logic level P-channel fet.
Diagram for reference. P2 connects to the sensor. It's IR LED connects via 500 ohm (~7mA).
Leo..
Yes its always been the case in switchgear use, always switch the positive and load goes to negative. Purely because if the bulb or relay (load) needs changing you would have one pin live, as you can switch (isolate) it.