Just adding my experience as I migrated my cable VOIP landline about a month ago to a very similar setup.
1) I use GMail as my primary email. I also have GV number associated with that account. For my home line, I decided to create an entirely new GMail account and assign a separate new GV number with the new account. I wanted to keep my primary email account and home phone accounts separate. I used the secondary account for the porting and eventually for the Obi setup. The GV number on the secondary account was replaced by my ported number.
You mentioned porting with a T-mobile SIM. Just to clarify, I think you will also need a T-mobile phone since you will need / want to test inbound / outbound calls during the process. At least that's what I did. I purchased a Kyocera Rally from Best Buy for $10 that came with the SIM and 30 minutes talk time. Looks like Best Buy is now selling the Kyocera for $4.
2) I did the same here. I use GV for all outbound calls and CallCentric for 1) Inbound Calls + CNAM and 2) 911 service. In my case I signed up for CallCentric's "Free Phone Number" service and received a free NY DID phone number. When you sign up for that service you will pay a one-time $1.50 setup and $1.50 for the first month of E911 service. You will now have two separate phone numbers to manage 1) GV and 2) CallCentric
The way I manage the two numbers is I have my CallCentric account setup on SP1, and GV setup on SP2. I have SP2 flagged for as primary for outbound calls. In order to get CNAM working for incoming calls, you need to go to your GV setup and add your free CallCentric number as a forwarding number on GV. That way when someone calls your GV number, then routes to CallCentric to pickup the Caller ID Name (CNAM) information and rings your CallCentric account on SP1. You never need to give out your CallCentric number, you just need to have it as forwarding number on your GV account to pickup CNAM. Without this, the calls to your GV number will still ring on the Obi, but not receive CNAM
Just an FYI, the reason I chose to setup CallCentric on SP1 and GV on SP2 is that with the GV forwarding, all calls are technically coming in on the CallCentric account. When I had CallCentric setup on SP2, I got weird stuttering phone ringing on incoming calls. I assumed that was due to the SP2 ring profile. I just swapped the two and now I get normally sounding incoming phone rings.
3) See above for CallCentric service names. The minimum cost you can expect to get it working is the $1.50 setup + $1.50/mo. As mentioned above, you can also add minimum outbound calling for $0.45 more. Just note, that in order to use that service you will have to dial "**X" depending on which SP you use for CC to tell the Obi you want to use a different service for outbound calls on a call by call basis, assuming you setup GV as your primary service for outbound.
4) I only use GV as my primary telemarketing / spam call filter. So far, I'm impressed. It is filtering out 3 - 5 spam calls per week that would have otherwise come through. And all of my legit calls are coming through. You have to make sure to turn on "Global Spam Filtering" in your GV > Calls settings page.
Hope this helps. Let me know if you have any further questions.