About a): Sarkozy (and please don't take that as indicating my support for the guy) did briefly state a position of not being ready to stay at ANY cost.

It did not destabilise anything other than Merkel, who immediately softened her stance -probably the only time Sarkozy had any impact on Merkel.

As for Podemos, whether they can do it like that in Spain I don't know, but certainly they would at least need to campaign on a platform of leaving, and there are polls, so people would know if they are about to win. So hardly a stealth action...
In France, it would require either a referendum or convening both chambers and getting a supermajority. Again, not something you can do quietly.

You are ruling out a) on the argument of a destabilisation from a lack of total commitment, to then promote a much, much stronger "lack of commitment" - in fact, commitment to leaving. That seems slightly odd.

As for ignoring the rule 123, yes, I'm all for that, but I don't think it's enough in the long run if the ECB won't act as a proper central bank, and it does nothing (or at least very little) to remove the fundamental imbalance created by Germany not playing fair.

Earth provides enough to satisfy every man's need, but not every man's greed. Gandhi