"...what is the relationship between cylinder 1 and all cylinders misfiring?..."

"Multiple Missfires" doesn't necessarily mean ALL cylinders are missfiring.

Since the plugs were recently replaced, that would probably be my first suspect. It wouldn't be the first time that new plug(s) went bad prematurely. Also, are you sure the correct kind of plugs were installed? I'm not at all familiar with your model, but in most other Volvos I've owned I had the best luck with the $2.00 NGK copper cores.

You could try a new set of plugs, carte blanc, just to see what happens. Or if you want to experiment, you could move the #1 spark plug to the #2 cylinder and move the #1 coil to say the #3 cylinder. Then clear the codes and run the car. See if the error code moves to cylinder #2 or #3.