Questions about Klipsch R-100SW

Yes, that should work, the Klipsch has Cinch Line In, to which the mono output signal of the amplifier is then connected.

Is your R51M really active? I only see normal speaker connections on the R51M. The subwoofer is connected via the Cinch input. Your speakers should therefore have a cinch-out connection. I only have such a connection on my stereo system or amplifier. A subwoofer out connection

It is not possible to connect them in series. However, you can connect it to the r15pm with an amplifier.

This is an active subwoofer. It does not have an Ohm rating as it is self-powered (via the supplied mains cable) and does not draw power from an amplifier (no connection with speaker cable). You can only connect low level inputs to it (for example 2 x RCA red and white from pre out, or an LFE cable from subwoofer out). In contrast, there are passive speakers/subwoofers to which you can connect high level inputs. These then have resistance specifications (ohms).

Mono is perfectly adequate. I have always connected it that way.

In what way is it an additional device? The subwoofer is connected to the AVR like everything else.

On picture 3 you can see that a connection cable is included. Unfortunately, I do not know how long this is. In my opinion, it should also work with a standard speaker cable.