I've done some investigating and can report back the following:
I have in the past changed which toolbars are visible since upgrading to Opus 10. I've changed them again today and toolbars.oxc continues to show the line on both my main and on the USB version of Opus, even after I've made a change to which toolbars are visible. So there's one odd thing.
Yes the config file changes and the changes stick.
Yes I did. There's definitely only one copy of the config file on each device, I've just done whole drive searches. On the hard drive that copy is at is C:\Users\steven\AppData\Roaming\GPSoftware\Directory Opus\ConfigFiles but I got to it via the alias you gave me.
Now here's a really interesting fact:
My main install of Opus IS exporting toolbars.oxc correctly as you saw in my example above. Nevertheless when I open Opus USB version I am seeing the default layout. What I've just discovered is that after then closing and exiting Opus, toolbars.oxc is then re-written to reflect the default layout. Even though I've made no changes.
If I then exit and re-start Opus and make changes to the visible toolbars, my changes stick. Heck, I can even re-copy toolbars.oxc from the main install and have it open from USB looking just as it does on the main PC.
So I now have a work-around for my problem!