Don't forget the bond holders. They have to agree to eat it big (90% of them need to sign off on a new deal). Some of them are not idiots, and have likely hedged the potential for bankruptcy with CDS swaps.
Put bluntly, one of the key parties that must agree to a haircut in order to keep GM out of bankruptcy likely has protection in place that would pay them substantially more should ch 11 occur. The gov (IMO) has been trying to ease the public around an inevitable bankruptcy, by acting like it is no big deal, and continuing to funnel taxpayer money to GM that will likely never be recovered.
As for who the heck is buying, I can only think this is the gift of more taxpayer money, offering an opportunity for current shareholders to get *something* out if they're smart enough to sell on last gasp rallies.