In regards to 1.) you need to go to Edit > Device Setup and press the Control Panel button, a new little window will pop up. In there is at the top an option saying something about ‘allowing exclusive access’. Make sure that that option is not selected and then also other applications can make sound at the same time. I’m not 100% sure, you might need to restart Dorico or even reboot, please try it out.
Regarding 2.), in that very same dialog as mentioned before, in the middle is a list of output ports where also your headsets should turn up. At the front is a little tick box by that you select which output to use.
(I didn’t read that thread before because of it mentioning in the beginning of the post that it considers Mac…)
I could change those Device Control settings of Dorico 4. Only, this worked for … one time use. But now this is no option anymore: I can’t select my headphones in the Device Control. It’s just not there anymore. Allthough I am listening through my headphones right now…