Unfortunately I don’t have the solution to your problem but just wanted you to know I was experiencing the very same difficulties.
It seems that somewhere in the buddypress/bbpress plugin code they are calling the page layout from the theme’s default page layout, because if you change, for example, the default layout from “Content on left” to “One-column, no sidebar,” all buddypress pages (like the members page, forums page, activity, etc) layouts change as well.
It seems that if you try to manually change the pages via the page editor, nothing changes. The values are saved but the pages still pull from the default layout’s settings. I’ve played with the CSS and was able to remove the sidebars, but that ultimately effects all the pages, not just the select few I’m trying to change.
My guess is that only way to solve the problem is to manually edit where buddypress/bbpress builds each ‘category’ of page (like the member’s profiles or activity), but unfortunately I haven’t been able to successfully locate where.
If anyone else has experienced this issue and would shed some light as to how they solved it, it would be appreciated here as well.