It is true that these snapshot referendums are subject to fraud and hijacking, so the obvious solution would be for the EU to sponsor and supervise a formal referendum.

Dangerous territory to be sure for a wobbling collection of separate states like the EU, but no other way in the long term to address the issue.

As with Canada/Quebec and Great Britain/Scotland, the Catalan separatists will probably find that pocketbook issues, and the natural human tendency to fear upheaval, trump tribal identity issues.