Is my analysis of quantum numbers describing the ground state of At correct?
Yes , there is another name for it (n+l) rule or Aufbau principle

http://en.wikipedia.org/wiki/Aufbau_principle

Even though it is not correct to check this rule at such higher values of Z where the trend sometimes changes due to very less differnce between higher values of shell and orbitals .But if you are a beginner you should not heed to this statement.