Please read this at CNBC regarding the potential value of the bonds to understand the reason the private equity bondholders agreed to the sweetened deal:
http://www.cnbc.com/id/31014489
Only a small fraction agreed. GM said 90 % must agree for the deal to go through. The piece in question says 67%. Only around 25 to 30 % have caved.
GM will not be able to pull out the deal and will file bankruptcy. The article says the bond holders will get a worse deal in bankruptcy. That is not so unless the court disregards the law. If that happens, there will be appeals all the way to the supreme court unless the bankruptcy judge comes up with a much more reasonable deal somewhere along the way.