The rules of the Euro, to which everyone signed up, prevent this, is what you meant. And you're telling me that I'm not bright?
As regards devaluing.... you seem to go along with Harold Wilson's " the pound in your pocket will not be affected" idea. Devalue, sell more, and the will be no negative consequences. Great. Except that only pushes the problem down the line. The countries get inflation. the import cost of raw materials for manufacturing goes up. Wages become worth less and soon the wages start to rise, the goods become more expensive and the prices rise, so another devaluation is necessary.
I think you over simplify the problem to suit your view. If it were that easy, Delaware in the states, which has structural problems, would get it's own currency and everybody would be happy. That will never happen because the States gain more by having a single currency. The Eurozone also benefits more as a whole than the negatives of a single country or some countries.
The Euro will survive, if there are closer financial ties in the Eurozone. Coordinated fiscal policies and a currency reserve bank as an insurance against a member having problems. Basically sharing the risk of a bailout if a country hits problems as you suggest.