I don't know if this is worth mentioning or not. So take it for what it's worth. The B2 will receive internet radio (through the vTuner service), but it does not do DAB radio. You'll need a different box for that.
You're right about the optical cable to feed the B2's digital output to your DAC.
As far as the BT router or Sky Q box is concerned, I don't know the answer to that.
I wouldn't be surprised if someone else will come along and answer this much more eloquently than I. Stay tuned.