[@robkk Moderator 10 years, 11 months ago]

1.

Would a user Login and Registration mechanism created by me for my WordPress site also work for the bbPress forum within that site?

Yes, bbPress is integrated with WordPress.

Would a participant-level user logged into my WordPress site be able to see the back-end?

Yes they can, but they usually can only edit their own profile. Moderators can moderate topics and replies in the backend too. Since their are different roles for WordPress and bbPress, you are able to have more flexibility with how you want to users to contribute to your site.

Can I restrict him only to viewing the front-end

You can disallow them from being in the backend by using a plugin like WP Admin No Show

3

Would plugins, like ‘Profile Builder‘, for simplifying the creation of WordPress Login and Registration mechanism also work for bbPress?

It should work fine with bbPress.