does a potential employer need to get behind the paywall before seeing profiles
Yes! No payment, no viewing of details for employers. They can do trial searches and see preview counts of results but no actual user details. And believe me, they pay .. uh .. a lot for the privilege of viewing actual user information, so this is not something that should happen casually.
Will ones employer be able to see names if casually surfing the site? Will the see other information?
No, they must pay the (much!) higher employer subscription rate first.
Now, if you have paid to file a Stack Overflow Careers CV, and your employer just happens to sign up for a paid Stack Overflow Careers employer account, too .. then yes, they'd be able to find you. It's sort of a catch-22 but I don't know how to get around that, since the same problem exists on every other job site that I can think of..