The "questionable/doubtful/unlikely" proposition always surprises me: The EU basically wants the whole of Europe to be part of the project to a greater or lesser degree, preferably as full members (if only for reasons of simplicity). To suggest the EU would put obstacles in Catalonia's, Scotland's, etc. way regarding membership is to me not realistic - the UK pressed the "threat" of Scotland ending up on its own outside the EU, and very effectively too (independence referendum campaign - the fear undoubtedly set in among significant numbers) but now the UK will supposedly be much better off on its own outside the EU - what the UK would have done had independence come about can now only be guessed at. Maybe Spain would/will in the same way obstruct Catalonia's membership but that seems to me likely only to be a temporary setback.