1. If you change the Author Registration Close date to a past date (From User -> Conference Site Management -> Conference Timeline), users will not be able to register with the site. In addition, you will have to ensure that individual registration close dates for registration types correspond to a past date.
2. If you have controlled access to accepted papers, these papers will be restricted to registered users only until the date you set here, at which point they will be freely available online. Please note that if you are testing this feature, all management roles of a conference (track directors, conference managers) have access to controlled content whether they have been registered or not; also, you must ensure that you aren't trying to access restricted content from an IP address that has been entered into the registration page.
2a. Yes, that field is to set a date to close comments on papers, if you've enabled them (you can do so from User -> Conference Site Management -> Reading Tools).
3. Yes, a user can add and remove themselves from the reviewer list at any time by visiting their user profile and checking/unchecking the reviewer checkbox.