Why is the half-life of 178m2 Hafnium isotope 31 years? Maybe it is somehow related to its nuclear spin (16+)?
I know next to nothing about nuclear isomers but I found this paper: Theoretical Assessment of 178m2Hf De-Excitation. It's third section describes the physics of hafnium isomers and on page 13 the authors write:
The decay of the $16^+$ state is highly suppressed (with a 31 yr half-life) not only because a change of $K$ by at least 8 units is required, but also because the transition has to have a high multipolarity $λ$, due to large angular-momentum differences between the initial ($J= 16^+$) and energetically feasible final states.
To understand this in detail I would recommend reading the entire third section.