1. Simply utilizing a plugin like Theme My Login plus out-of-the-box capabilities, this should be pretty straightforward:
But, if you find additional capabilities are required you may want to think about upgrading bbPress to BuddyPress (http://wordpress.org/plugins/buddypress/) and/or use a members plugin like:
2. They won’t, their role would need to be as high as Editor (which you would have to explicitly grant them that level of permission) in order to edit pages. However, for peace of mind, you can set Theme My Login to block everyone except for Admins from being able to access /wp-admin/. Once someone gains access to view a password-protected page, it’s no different than their viewing of one of your normal, public pages.