I still do not understand how you can 'negotiate' a common currency amongst nations.
The biggest problem is not the currency itself (aside from it not having a treasury) but things like the Stability and Growth Pact, and the matter of the interest rate regime being bad for a great swath of the nations in the eurozone (especially the Club Med members).
Usually when we read of EU countries balking at the state of the monetary union is comes from three members - Germany, Austria and Finland.
All allies during WWII until the Finns cut loose when Germany was in full retreat on the Eastern front and the Soviets made a renewed push into Finland in 1944, forcing it to give up the territory gained in 1940.
These three are still alligned - two by language and culture and two by a common distrust of Moscow.
The Lisbon treaty does not allow for a member country to leave the EMU and stay in the broader EU (old EC). Finland is not strong enough to survive outside of the EC so it’s not going anywhere unless the Germans say when and how.
You dangle the bait of currency Manipulation to those in power wherein the can foll around with rates and such and line their pockets. Like we allow the Banksters here in the USA to do.