Those are all valid points. But why didn't Spain support a referendum like Canada supported Quebec twice and the UK with Scotland? Like the OP said, this could all have been stopped if Spain had supported the vote and campaigned for the no. How could Catalonia get a 'legal' vote?
I'm under the impression the only way an independence vote could be legal is if the Spanish Constitution was changed to allow that. The current constitution declares Spain "indivisible", with no more specific details. That would seem to pretty clearly exclude any possibility of a part of Spain legally separating from Spain. The UK's legal system is a little slipperier. UK doesn't have a hard and fast Constitution. If Parliament allows something, it's basically legal (less hard checks on legislative power than your typical constitution).
It's interesting how this stems from a cultural difference. In very general terms, the UK is based on common law, in which we decide the truth as we go, based on the decisions of parliament and court cases. Spain and France are based on civil law, which are much more rigid and top-down, but also more clearly defined. In civil law, if it's not explicitly declared illegal, you can do it. In common law, it's a little murkier. This even seeps into the language (The French say they "have the right" to do something, the English say they are "allowed" - active vs. passive).
Of course there is great overlap between the two in reality, but at their core the two legal systems have different philosophies. Hence why you end up with situations where the UK may be more flexible about things like separation.
Actually our Constitution states that Spain is indivisible for a reason. We had some rebel "cantons" in XIX century and a few revolutions in XX, so when we voted a new Consitution in 1978, it was decided to secure the union in the highest law. Of course you can modify the Constitution and there are legal ways to reach a consensus for that. It's just more difficult than holding an illegal, rigged referendum because some rogue regional politicians think they're entitled to it.
Well it is not so easy.
First, they will need to modify the constitution, which requires being approved a qualified majority not a simple one in parliament, in this case, is 3/5 of the parliament this reform might need to be approved by referendum (all the country not just Catalunya) in case a tenth of the parliament requires it.
Second, the government has no desire to modify the current regional system, which is in line with its voters. Anyway the Canada case is not the norm and similar legislation are in use in other countries, for instance in Germany.
https://www.thelocal.de/20170103/bavaria-must-remain-part-of...
Except, Germany previously did allow regions to leave.
There was a huge political situation after WWI about that, but in the end 5 countries agreed to allow the counties in the north of Germany to choose if they want to belong to Denmark or Germany.
Second, the government has no desire to modify the current regional system,
Sorry, I need to correct that. Not just the govern, an overwhelming majority of citizens in this country have no desire to modify the current regional system. At least in the direction that separatists want.
Stage one would be amending the Spanish constitution to get rid of the notion of Spain as being indivisible, and that's where it becomes really tricky, as you then need to get consensus from all of Spain to allow that.
David Cameron? It's perhaps fair to say he called the referendum expecting to win, continue his political career and quell the infighting within the Tory party and achieved none of those.
While Madrid's handling of the more recent Catalan independence campaign has been pretty inept the Brexit vote sets a pretty strong counterexample to "just call a referendum, you'll win, it'll be over for 20 years". Scottish independence is a good example too, that's hardly gone quiet...
