You can use an analog fax machine with an OBiHAI device to send and receive faxes whether your VoIP provider is Google Voice or someone else. A fax machine is just another type of analog phone to an ATA.
You won't be able to use Google Voice with an OBiHAI ATA after May 2014 - you'll need to use another provider such as Callcentric, etc.
911 service will not work if Google Voice is the provider you're using. Other VoIP providers offer 911 service, either included in their service or as an add-on feature.