Just commenting on the GV aspect as it relates to OBi use:
I think there are too many "balls in the air" to troubleshoot this -- I'd start by diagnosing a plan, vanilla configuration of the OBi plugged into the primary internet router, with an analog telephone plugged into the phone port, and testing if two-way audio works on both outbound calls (which are using Chat/XMPP directly with GV's infrastructure), and with inbound calls, which you are forwarding to as SIP DID. If both work fine, then move onto the next stage of complexity. If inbound calls work, but outbound calls have one way audio, then we can troubleshoot that, for example.
One reason I suggest this, is that there have been some recent (in the past week) issues with the audio stream dropping on some users' GV calls. Google Engineering is investigating that issue, which could be a red herring, confusing your troubleshooting.
I also don't profess to be the VoIP network architect/expert, so I will leave that conversation to y'all. I will just remind you that GV doesn't use SIP to set up or control call signaling, so don't bother futzing with SIP settings for GV SPs. I don't know how the old Chat/XMPP service works or doesn't work with ICE, STUN, NAT or other network traversal techniques, sorry. One other thing, don't forget to disable SIP ALG in your router, if your calls via your SIP DID are the only ones having one-way audio.
Regarding multiple calls using GV: You can only register and use one GV account per OBi SP, so on the OBi 202, that gives you support for a maximum of four GV accounts/numbers. GV supports call waiting when forwarding calls to a conventional telephone carrier or SIP DID, as long as that carrier supports call waiting. I don't know the maximum number of calls that can be juggled this way; it's not documented.
Assuming that you have multiple GV accounts, and you want to use them with your OBi, you would log into the OBiTALK portal page, with your OBi device added, while also being signed into the desired GV account on the same browser session. You'd then simply repeat the GV setup procedure for each OBi SP you want to assign to a GV account. The procedure will even display the GV account it's about to connect, to make sure you are selecting the right one (it's easy to accidentally select the wrong account, since Google supports multiple accounts being signed-in on one browser session).
You probably already figured it out, but just in case, here are my instructions for setting up GV:
http://www.obitalk.com/forum/index.php?topic=8560.msg56460#msg56460