I’m not sure I completely understand what you are asking either. But if I assume what you want is to show a Group’s state on MainUI but send a command only to one specific member of that Group I can offer the following.
First none of this has anything to do with the semantic model. So just avoid going down that though process.
What you are after then is to have a widget that shows the state from one Item (your Group) and sends a command to another Item (i.e. one of the members of that Group). That’s easy enough. You have almost full control over how the widgets are displayed. So you can very easily show the state of one Item and configure the Action to command a different Item. A lot of the more interesting widgets on the Marketplace will combine the display and commanding of lots of Items into one widget.
To define how the Item appears in the Overview tab cards, you’ll need to set the Default List Item Widget on the Item.