If Catalonia declares independence (today, as you said) then Spain will stop it and apply direct governance there, ignoring the declaration and the Catalan government. They will do so because the EU will not recognize Catalonia and will support Spain to avoid it. Catalonia will not have the power to do it or they can enter into a conflict (but that is almost impossible, nobody wants it)
See my answer in this question: What could Spain do legally if Cataluña proclaims independence?
EU has the opinion and will not recognize it, and therefore Catalonia will not be so willing to declare independence in a situation like this. Catalonia wants to split but with more or less good terms. EU will not change anything, but if some day they really split from Spain on 'good terms' then there will be more regions that will also complain. One of them being Basque Country, that as you said will push in this moment to Spain to also secede.
The Commission statement also supported Madrid’s line that the vote, which Catalan leaders said recorded a huge result for independence, was “not legal” under Spain’s constitution.
A spokeswoman for the Socialists and Democrats Party said the Spanish constitution needed to be recognised and respected.
She said: “The European Parliament as representative of all European people is calling all parties to sit down and work together for a peaceful and responsible solution, in the framework of the Spanish constitution.”
It is just the latest warning issued by europhiles in a bid to first block, and then ignore the referendum
They do not find legal the referendum and therefore a declaration of independence
About the veto, Spain is still part of one of the nations voting and any government in Europe wants any region to split. It is not true that any EU country does not have a region with these feelings. Germany has Bavaria, now is not so strong the independence feeling, but after a 'Cataxit' this feeling will increase, and Germany wants to avoid this.
Too many countries in the European Union have secessionist problems, including the UK, Belgium and Italy and this is not a unique problem
Willem Buiter, chief economist at Citi