The solution is easier than your think....
Port your regular phone number to GV (through a pre-payed cell phone). Set up Callcentric for your E911 service and let Callcentric issue you a new phone number (ie don't port a number over). You will forward you and your wife's GV number to that Callcentric number. If you set up a new GV number for your parents to use, then forward that number to the Callcentric number too. Or you can have Callcentric give you a number in the 603 area code and give that number to your parents. Then you can skip setting up a GV number for them since they can call directly to the Callcentric number.
You'll have to pay Callcentric for these incoming minutes, but only for the forwarded calls from your cell phones and incoming calls from your parents - all other incoming and outgoing calls from your GV Home number will be handled through GV for no charge. You can pay by the minute ($1.95/mo + $0.015 per minute) or if you expect to have a lot of incoming minutes (break even is about 265 minutes/mo), they have an unlimited residential incoming plan for $5.95/mo. Check out the pricing here
http://www.callcentric.com/did/In other words, your original plan works just fine, but instead of forwarding your cell GV numbers to your new home GV number (which you cannot do), forward them to the Callcentric number that you are going to be setting up for E911 service anyway. You don't need to give this Callcentric number out to anyone (other than your parents if you don't use GV for that setup) because only GV numbers will be forwarded to it. People will have your cell GV numbers and your Home GV numbers as they always have.
Your Obi110 will be set up with your Home GV number as service provider #1 and Callcentric as service provider #2. All non-emergency calls will go out via the Home GV number and people will see that caller ID on their phone when you call. 911 calls will go out via Callcentric to use their E911 service.
Incoming calls from your Home GV number will come in via service provider #1. People that call your Cell GV and your wife's Cell GV number will have those calls ring the respective cell phone number AND the Callcentric phone number. This means both the cell phone and the home phone will ring because those calls will come into the Obi via service provider #2 (the Callcentric line). Callcentric will forward the original caller ID number so the actual caller's number will appear on your home phone's caller ID, not your GV number or Callcentric number.