I am no expert in all these but...
1. After May 15th, Obi device will not work with GV. Which means no incoming or outgoing calls using obi+GV. So no you can't use Obi to call international via GV.
2. Preferred is that you either port your GV# to CallCentric, Anveo,
Vopi.ms, Future9, PhonePower etc. And use them to call USA/Canada. For international calls...Voip.ms, Callwithus, Future9, CC, etc all offer calls to India for about 2.5cents. You can use SP1 to configure to use VOIP provider that gives you best value for USA calls and then SP2 for calling India. I use CheapVoip to call international and have set them up on my Obi.
3. If you don't want to port your number then get a DID from any of the above mentioned VOIP provider and forward calls to that # from GV. But note that you will not be able to use GV for outbound. I had GV forwarding to my Anveo# but there were lot of problems. GV is not 100% reliable in call forwarding.