Glad to be of help, and let me heap on a few extra points to your response and your understanding of how 911 services operate.
If you're able to strip down to a 911 only landline connection, there will be no phone number and no account connected to the service. If the call is disconnected, the operator has no means to ring back. If the person calling cannot provide accurate location information over the phone, they will have great difficulty locating and dispatching first responders.
Inversely, in order to make 911 services work on VoIP due to its nature, the
E911 extension was required. This extension allows VoIP users to set a home area for appropriate
PSAP routing, and automatically provides and sends the caller's name, phone number and address on record to the answering operator when 911 is dialed. It should work well and without problem. E911 is also the method by which cell phone triangulation information is sent to dispatchers, as well.
If there's still concern, pick a reputable VoIP provider that offers proper E911 support (which should be everyone on the Superguide recommended list, IIRC) and during the trial period call your local police dispatch office, express your wife's possible concern over E911 effectiveness, and request to schedule a 911 test with your local PSAP to ensure the correct information is being received on their end. Good UPS + VoIP ATA + demonstration of E911 effectiveness = soothed landline 911 paranoia.
I've had to use 911 on our VoIP line a couple times, and the usage experiences have not shaken my faith in the system. Frankly, there's far greater concerns to the reliability of the 911 first response infrastructure than VoIP vs. Cell vs. Landline. The call center connection is the easy part...