Also, did being a member of the EU give any benefits with regards to government debt, and if so, what were they?
It's not that it couldn't default on its debts. It's that defaulting on its debts would have had a cascading effect on the European Union (EU). So the EU kept bailing them out just enough to keep them from defaulting on their debts.
Another reason why the EU bailed out Greece is that many of the debts were owed to citizens of EU countries outside Greece. Since Greece debt was denominated in euros, it was easier for EU investors to buy than if it had been denominated in drachmas. If Greek debt had been mostly owned by Greeks and the rest of the world, there would have been less pressure on the EU to bail them out.
And if you want to argue that Greece could have avoided austerity with a default, remember how it got the debts in the first place. It was running a persistent, structural deficit. A structural deficit is one where even if it magically eliminated its debt payments, it still didn't have enough revenue to cover current expenses. It needed the bailouts just to be able to phase in austerity. Without them, austerity would have been worse.