Keep a few points in mind here, because this may have changed in GV since those 2011 and 2012 issues were described and reported.
You have one Google account that has a Google Voice number and Gmail (you can obviously have more than one google account, but for this description we're talking about one). The Google Chat checkbox (as a forwarding phone) is in the GV web page, under Settings > Phones.
You can make an outgoing GV call from either:
a) The GV webpage, which 'calls you' to 'connect the call' in your Gmail web page if your Obi is disconnected, and if you select the "Phone to call with" as "Google Talk". I'm gonna guess you cannot select Google Talk, you only have your other forwarding phones in the list. For testing you want the Obi disconnected.
b) You can also place a call from directly in your Gmail web page (again, the Gmail webpage and the GV webpage are two different webpages that you can and usually do have both open simultaneously). For this call you do not select the forwarding phone to use (to 'connect the call').
The test I was asking about is a), specifically test an incoming call. However, based on your predicament, try incoming and outgoing from all the above possible scenarios and see if the checkbox comes back to re-enable itself. This is just nothing more than an educated guess at this point. I hope it helps.