I take a othogonal XYZ axes system, with Z pointing in the direction of the arrow
1) Fe-I:
X-axis is a 4-fold symmetry axis;
Y-axis is a 4-fold symmetry axis;
Z-axis is a 4-fold symmetry axis;
So: this means theorem 2 is fulfilled and the EFG tensor is zero. This means also that all these axis could be Z-axis of the PAS and etha = 0
2)Fe-IIb:
X-axis is a 4-fold symmetry axis;
Y-axis is a 4-fold symmetry axis;
Z-axis is a 4-fold symmetry axis;
So: the X-and Y-axis and Z-axis can be used as a Z-axis of the PAS system with etha being 0.
3)Fe-IIa:
idem for Fe-IIb