There are at least three different stages of 737 rudder modification in service. It is important that you know which you have installed as they work differently.
It is my understanding that if you have the RPR and/or RPL installed rudder authority is reduced by 1/3 when above 1000ra on take-off or 700ra on approach. So full rudder authority is not available at high altitude.
If you are in an NG you have load limiters instead of the RPR/RPL, these reduce rudder authority by 25% at blowdown speed. The LL’s become active above approx 135kts with no altitude constraints. So full rudder authority is available at any altitude if below 135kts. This may catch your spinning scenario.
Bringing this back to Vmcg/Vmca, does this mean that the 737 has two different Vmca’s depending upon whether or not the rudder limiter is active or not? I suppose it must.