I recently started a try at this and I saw in the options menu that Expert mode is not disabled even though in my opinion it should be since  you're going for one specific achievement. I toggled it off to see if it's indeed clickable and immediately back on. Does that invalidate my achievement try? I didn't exit the menu with it togged off, it was a quick off-> on

The achievement won't be invalidated unless you close the menu and save the setting.

 

We allow you to opt out of Expert Mode at any time, but if you save that change you can't opt back in.

This means i'm safe right? since i could toggle it back on and it's on right now. if i had closed the menu the option would have been permanently disabled.