(2) If he performed it after the congregational prayer, will it be considered as Sunnat or Nafl?
(3) Will there be any harm in performing the four Rakats Sunnat firstly and then the 2 Rakats Sunnat?
الجواب حامدا و مصلیا
If someone leaves off the four Rakats Sunnat of Zuhr Salah to join the congregational prayer, then he should perform it after the congregational prayer. Even though it will be performed afterwards, it will not be considered as Nalf. In fact, it is still considered as Sunnat. As for performing these 4 Rakats before or after the 2 Rakats, both are permissible and there is no harm in neither of them. However, Alama Ibn Humam has preferred performing the 2 Rakats before the 4 Rakats.
And Allah knows best.