hello
part 1:
we can use single IR transitive module its for 1$, its can detect motion in 4 CM range, no light interference and it is made for short distance only
part 2
we can use parallel to serial ICs to send the data from sensor serially to arduino
anther way, we can do that using external circuit that send data serially, it will detect all sensors within one second
part 3
yes i can
i am expert with arduino, Rpi, and pic; but i prefer using arduino, to get minimum cost, and the server can record the time of receiving data.
if the cost for this part not impotent, we can add time and date unit and send the time with massage once states of sensor happen
note
the price include all hardware cost
the price did not include any shipping cost
need more data about part three """""" the content of massage" and" if tow sensor changed at same time would you prefer to send single massage, or each one separately"