I just set mine up in a similar way to what you are after. First, you need to subscribe to Premier. Without this, no 2nd line & no multiple numbers.
You will hook the fax up to the OOMA hub. They don't really work well from a Scout. Hook your phone up to a scout over the home phone line, as per the manual. After setup, in the Lounge you can set up a 2nd number. You can also associate the 2nd number to ring on just the hub. This way, your fax will pick up but none of your phones will ring.
The issue remaining is having a 2nd phone available. You could put a splitter on the fax line at the hub & accomplish it this way but when you dial out from the attached handset, caller id will show your fax. Also, that phone would ring during an incoming fax. Rather than doing this, I purchased a 2nd scout recently and plan on adding that via the household phone lines.
With this, I'll have 2 phones, sharing the same phone # that can talk simultaneously over the OOMA's 2 lines. Also, the fax is on its own phone # and will pick up automatically without the other phones ringing. Of course, only 2 of the 3 can work at a time but it sounds like this is what you have anyway.
With that being said, OOMA is meant for residential use, not commercial. Be sure to read the terms of service to see that you fall within their guidelines. They seem fine with small home offices but the service is not designed for full sized businesses.
Also, with regards to the fax, some have better luck with faxes over OOMA than others. You may want to search the forums for fax issues. While most can get a fax to work, it's not 100% reliable.