Happy Labor Day, folks! And guess what? I've been laboring!
Yes, I finally
got the PC up & running again; the FocusRite 4i4 is installed and connected; the Yamaha audio out is actually going out; and Mixcraft 8 is recording it! Getting them all to cooperate is a delicate ballet of the various control panels, settings and preferences (with the bewildering requirement to select Speakers as output and not Optical Audio, even though the optical audio cable is the only cable to the speakers) - but at least I'm at the point where I can select the various voices and effects on the Yamaha and record them in Mixcraft!
So far, so good - and it appears that the USB-C cable from the Focusrite is carrying audio as well. I had some noise and disconnected the audio cables... and still got audio to the mixer! So there's that.
Thanks to all for your help, and I hope to be able to check in after a couple of weeks to let you know that everything is running smoothly!