declared the independence of Catalonia with the consent of far fewer than half its citizens
We could apply this same logic to the EU referendum. As far as I'm aware, the EU referendum passed with the consent of far fewer than half of Britain's citizens.
Not really. Anyone who had an opinion on the EU referendum could easily go and vote; they had no reason not to. The Catalan referendum was illegal. Those who do not want to separate from Spain (actually likely the majority going by opinion polls and the last regional election) had to validate the process by taking part in it if they wanted to have their views count. To vote on Sunday, they had to go against court orders of a state they support, go along with the illegal scheme of politicians they don't support, and risk their personal safety to get to the polling station through crowds of protestors and police.
Many 'no' supporters would have chosen to boycott the whole affair, which unfortunately just allows the separatists to shout about getting a super high vote share for them. They aren't going to admit that it was illegitimate, that they are basically declaring they won a race in which they were the only competitor.
It's hardly comparable to the validity of the EU referendum and its low turnout. As it happens I think referendums should have minimum turnouts and majorities put in place in advance, so the EU referendum shouldn't have passed. But still, it is not comparable to the choices that had to be made by a 'no' voter in Catalonia.
And again, I still don't agree with the levels of police violence we saw, especially against people who looked like they were just walking out of polling stations.
But we non-Spanish should not be sticking our noses in and signing petitions like this. It's silly.