The child theme does not provide the no-sidebar template as an option! The only options are Default Template (i.e. including the sidebar) and Blog Template. It's an old theme (Patricia: https://9seeds.com/product/patricia/).
I looked at the code, and it appears that the default Genesis templates which include the no-sidebar option are disabled specifically.
I can remove all the widgets from the sidebar, but the space occupied by the sidebar is still there. so the content area does not expand to full-width as I would like.
I was hoping there were some way I could simply override this annoyance by removing the sidebar area with one of the Cobalt tools.