Yes. You can use that amp to power all four speakers. You would only need to run new wire to the speakers, OR, you could clip the OEM wires that go to the speakers and run them straight to the amp. For the front doors, you would clip them behind the radio. Tape off the wire that goes to the radio.
Yes, you just adjust the Gain on the amp to reduce the voltage.
It is probably running in bridged mode. That means both channels are combined to one, a very common practice. For this setup, you will only power the door speakers because they are the same model. The dash speakers can stay on the radio's amp (common practice). Remember that you are going to blend the door speakers with the dash ones. That way one set will not drown out the other.
Oh snap! I did not even think of checking on the amp model. You are spot on, it is a mono amp.
