Hi there!
You've got several questions. Regarding using two different inbound telephone numbers, if you are porting the GoPhone number into the same single Google Voice account that already has a Google Voice phone number, then be aware that this is not equivalent to having two different lines of service. One number will become the primary inbound number, and the other will become the secondary inbound number. Whichever number is designated as primary will be the one displayed as your caller ID on outbound calls. You can swap primary/secondary number roles on the GV Settings page after the port is completed. If your first number was a freebie from Google, you'll have to pay an additional $20 to keep it as permanent; otherwise, it'll be deleted after 90 days.
All inbound calls to either number will ring the same forwarding destinations (Chat/OBi, and any 10-digit telephone numbers you've added as forwarding numbers).
So: that setup isn't ideal for your voice/fax use case. Instead, you'd want to have the voice call number on one GV account, and the fax number on a different GV account.
Have you already submitted the number port request? If so, you may be able to cancel it. If it's too late, then you'll need to complete the port and then take some steps to move one number to another account.
Then, you'd set up the OBi device with GV #1 on SP1, and GV #2 on SP2, thus creating two completely different lines of service. You would then map SP1 to the 202's phone line 1 jack, and SP2 to the 202's line 2 jack.
Regarding security: yes, GV on OBi devices no longer stores account passwords. During the setup procedure, using the OBiTALK portal, OBiTALK will send you to your GV account, where Google will ask your permission to share just the Chat function with OBiTALK. When you agree, a secure access token will be exchanged with your device. You can revoke this permission any time via your Google account dashboard. This does not grant access to email, for example.