1. Once the feature is enabled, Conversations Inbox Opt Out will appear on your personal Settings page (top right Settings in old UI, or Account > Settings in the new UI). It's placed below Ways to Log In, and above Web Services, on the personal Settings page:

2. The Inbox appears in Global Navigation as usual.
3. Yes, with the feature enabled, you can still click on Inbox and see the usual initial display of read and unread messages across All Courses.
4. Ask Your Instructor a Question remains even with the feature enabled.
5. Yes, by masquerading, any Admin with Become permissions will be able to see and change the Conversations Inbox Opt Out checkbox state on behalf of another user.
CAVEAT: I can now confirm that the Conversations Inbox Opt Out feature does not currently affect or control display of the unread message count on the new UI's Inbox badge. I have an on-hold case with Canvas Support about this problem. As a suggestion, anyone else affected by this issue might also want to report it to Support mentioning JIRA issue CNVS-29400.