I think for them it's only about having their own constitution and sovereignty. Another one of their goals might be to stop using the Spanish language, which a big part of the population has done already.
They are perfectly fine with open borders, being a part of the European Union, using the Euro as currency, and even with keeping their Spanish passports (i.e. double nationality).
I'm sure they would, but once they are independent, they can't legally use the Euro. Would they even continue to be Spanish citizens and be able to travel freely in the Schengen area? A new state takes years to join the EU (assuming the EU would accept). How about sovereign debt?
All these (really important) issues that should be part of an extensive debate so that people could understand better what is expecting them on the other side.
Also, I don't know any country that allows constitutional changes to be done via a simple majority in parliament (they had 3 votes above simple majority).
The only reasonable exit at this point, I believe, is to have new elections.
Of course they can continue to use the euro. They wouldn't have any control over it, but a country can use whatever currency their population will accept. IIRC there are a few small nations that don't really have their own currency and just use USD instead.
You are right, I oversimplified it in my comment. People could still trade using euros. But I guess the only way to get more currency in the country would be via foreign trade. Banks would not have access to EU QE, deposit guarantees might be no longer valid and the state could no longer print currency to replace old ones.
They are perfectly fine with open borders, being a part of the European Union, using the Euro as currency, and even with keeping their Spanish passports (i.e. double nationality).