We have, in unusual applications (not for Parkinson Dz), used DBS settings as high as 9 volts. This exceeds the recommendations of the device manufacturer, but we have seen no adverse effects from this. So I don't think your husbands high voltage settings are necessarily dangerous. On the other hand, we rarely use settings higher than 4.5 to 5 volts to treat Parkinson patients.
One fundamental principle in this regard is that the energy required for effective DBS is a function of lead location. In general, if the lead is optimally located in the brain, less energy is required to produce the desired effects of DBS. If one of our DBS patients is requiring very high voltage settings, we begin to worry that his/her lead may be suboptimally placed. In other words, perhaps--despite our best efforts--we missed the best location when we implanted the DBS lead. We measure the lead locations in all of our DBS patients with post operative high resolution imaging and the truth is, we miss sometimes. Occasionally, if we are unable to produce the expected effects with DBS even at higher voltages, and our imaging suggests that the lead is suboptimally placed, we do the surgery over again, removing the ineffective lead and replacing it with a new, hopefully more optimally positioned lead. We hate to do this, but we have been quite successful salvaging "DBS failures" with this approach. The bottom line is that your husband's lead may not be in the perfect position if higher voltages are required. If he is getting good benefit with these higher settings, however, it is probably not worth considering surgical replacement of his lead.
In response to your second question, many patients with Parkinson disease never develop tremor. Idiopathic PD comes in two general types: tremor-predominant and bradykinetic/rigid. Those with bradykinetic/rigid type PD (e.g. your husband) may have little or no tremor.
Best of luck to you and your husband.