You would want one left and one right, here is a cable to do so
Yes, it will act as your dac and if you choose also include the audio from the interface via direct monitor
With the focusrite you cannot do this, it will be a fixed volume from the incoming mic. There are other interfaces that have this feature though
What you would have to do is connect those speakers to either the back of the jds labs el amp ii and use the el amp as a volume control for the speakers, or instead split the output of the scarlett and then control the volume form the interface