I cant see the writing, but the rca is most likely one pair is the input, and one pair is an output to hook up another amp.
Yes just leave the wires hanging there. Just make sure you cut the ends off them so no bare wires are showing.
You set the input sensitivity to what your deck has for output voltage. If it has 2 volt or less, use the 2 volt or less. If you got higher than 2 volt outputs use the higher setting. Just look up your deck and see what it says it has for pre-out voltage.
High level input is for wiring it into speaker wires for a signal source. This would be used for a stock stereo in the car that would not have rca pre-outs on it.