Currently it is not possible to buy a theme on the marketplace, write a own theme thats inherit on the bought theme and create duplicates in the admin for each sales channel.
The duplicate icon is disabled because the inheritance would not work.

I think not everyone will work on the SW6 default theme. Not everyone will write a own theme based on the default. There is also the way that you want to buy a good theme, which many configurations and features. Thats saving alot of time. But you might need to apply some adjustments.

Thats possible but at this point you are not able to create duplicates in the admin for each sales channel to set different config values. Workaround is, to ignore the theme system and write a plugin that is adding custom SCSS and extending templates.