The screws are anti-tamper torx and they are available althouth the latest AMM have a new 5 sided unit that I've never seen the tool for. There is no need to unscrew the capsule portion for replacement though the whole assembly is sold as a unit.
Your diagnostic technique would be real appropriate if you take a 5lb hammer and smash it. If it changes the way it runs replace it
That I would agree with. If you have an inclination for testing with meaning see what the Key on engine off output is. A good unit is 1v. Further testing would use the adaptive values from actual values.
The forward O2 sensor is the primary mixture feedback the second one is mostly for evaluating the efficiency of the cat although there is a secondary mixture control available at times, not well defined.