I agree with QBZappy on (2) and (3), but have some additional comments on (1):
It's not possible for a fax machine to know that an incoming call is a fax, unless it (or another device on the loop) first answers the call. That would unfortunately have the effect of "stealing" a voice call that came in when the first line was not in use.
However, what you are asking for seems pretty strange. Unless you have a special reason for wanting the same number for voice and fax, I would use two separate GV numbers; one would ring the voice line and the other the fax. If your fax machine can recognize and answer a "distinctive ring", you could set it up so voice calls would also ring the second line, which would have the both the fax machine and one or more phones. That way, you could also make or receive a voice call while the other line is in use.
Going a step further, I'd use a fax-to-email service to receive faxes, which has many advantages over using the machine. You can get free fax numbers from Callcentric (NY),
k7.net (WA) or VoxOx (CA). Or, get an Anveo Value DID in your area for $0.99/mo. All of these are IMO more reliable than GV through the OBi. If you have paper documents to fax, you could still use the machine for sending.