1) No. You can not get the raw unmodulated signal.
2) I have never been able to get the received power level to work.
Following the RF24 library sample code for the Arduino never worked for me.
Don't know what the trick is.
3) The nRF24L01+ will not work as for radar. It does not detect itself.
Even if you used two, the power detect is line-of-site between them.
There is no bounce/echo detection as you see in a normal radar.
(Welcome to the forum.)