H bridge issue

I don't think the voltage rating of your motor has much do to with the issue, at least in a direct sense.   That the addition of the zener didn't help strongly suggests (though doesn't prove) that excess Vgs isn't the issue; zeners still have a finite impedance, and it's possible that there's a transient current sneaking through somewhere that's popping your FETs. 

Do you have a 'scope available?  Problems of this sort are a lot harder to solve without one.  If you have a resistive load  (prefereably non-inductive, or at least much less so than your motor) available that'll handle a similar power level as your motor, see if driving that gives you the same results.  If the problem goes away, that would suggest that the problem likely is related to the dV/dt that arises due to shutting off current flow to the motor's winding inductance.  If not, take a close look at your control inputs.

Since you're not seeing a huge temperature rise prior to failure, violation of some voltage parameter seems most probable.  You might want to try adding a bit of impedance back into the gate at turn-off, in order to give you a bit less dV/dt at the output nodes by slowing the switching transitions.  Yes, you'll get a bit more switching loss, but it's better than blowing FETs all the time...