There are a number of threads on the forum that discuss how to make a field mandatory (eg. viewtopic.php?f=3&t=5950
); but of course, you'll have to modify the check so that it only checks when a user is registered as a reader; and you will also have to ensure that this applies when a user is editing their profile and checks the "reviewer" role for the first time and saves the form. Take a look at the above thread, and let us know if you have any further questions about that point.
Regarding your second question: it is indeed possible to unenroll as a reviewer when you have an active review assigned to you; this should not be possible, and I have filed a bug report here
. Please feel free to CC yourself to that report for future updates.