the collapse of the US dollar wouldn't put much strain on the union if the Euro becomes toilet paper it will spur addition exits probably.
I'm pretty sure I've still got some French francs tucked away at the bottom of a box somewhere. Never felt right to just throw a currency in the trash. I guess I'll hold on to them in case suddenly they become valuable again. 
and in this yes, every country has a veto right when it comes to this. It takes one parliamentary majority in one to refuse it and the project is dead. Not even the EU parliament has power over this decision.
Interesting. It's probably too soon to know, but have any countries expressed any objections to a CBDC so far? And on what basis? Surely when they get fed the usual nonsense of "but think of the children" and/or increased tax revenues through less tax evasion, they'll be quite happy to comply.