1. Correct. In your case, you should use the RCA version.
2. The 6db lost is only meaningful if your power amp is truly balanced and has both RCA and XLR inputs. If your amp only has RCA input, there is no other way to connect it so you will never have the 6db gain.
3. I don't know if the switches of the Model 5 crossover changes its impedance or not but you set those switches on and off to vary the crossover point for a particular input impedance of your power amp.
2. The 6db lost is only meaningful if your power amp is truly balanced and has both RCA and XLR inputs. If your amp only has RCA input, there is no other way to connect it so you will never have the 6db gain.
3. I don't know if the switches of the Model 5 crossover changes its impedance or not but you set those switches on and off to vary the crossover point for a particular input impedance of your power amp.