Eventually this sort of thing will be much easier because we intend to extend the concept of the Playback Template beyond the current ‘Default’ one, so that for instance you’ll be able to set up one of your typical orchestra configurations in a score, and route all the instruments to your choice of plugins and assign expression maps, and then you’ll be able to export that configuration as a Playback Template that you will then be able to import into other projects.
Load Sounds for Unassigned Instruments is specifically for cases where you may have instruments in the score that don’t have any routing data set for them. In that case it will assign a (HSO) patch, but crucially it won’t change any other sounds that you have loaded. A key part of Dorico’s design concept is that it won’t make destructive changes to the setup of plugins that have been loaded unless you tell it to (which is why if you delete an instrument you’ll have a gap in the mixer). If you choose to Apply Default Playback Template then it will overwrite any changes you’ve made to plugins and get you back to a default state.