If a coalition headed by New Democracy and other parties that say they're committed to the euro (and to finding a way to keep the bailout deal) wins, that government would face a huge problem. Another 10 billion in austerity cuts would bring even more Greeks into the streets. The country might indeed become ungovernable. The government might fall almost immediately. And Greece would be headed out of the euro in August anyway.
After August, I think the Greek economy sinks ever deeper in to recession, the country cannot meet even looser austerity demands, and it heads out of the euro anyway.