yes if they leave or are forced out of the euro they would go back to their own currency although I guess it doesnt have to be the drachma, they can call it what they want
not necessarily. Some countries that use the euro aren't EU members (Kosovo, Andorra and others), so Greece could conceivably leave the union but keep the currency.
It's unlikely to be kicked out, though; it's almost impossible to do this, so it would have to resign voluntarily: