This is complete nonsense. The Spanish state has outright refused for a legal referendum to be held. This is a denial of democracy. The Catalan regional government has attempted the best possible democratic means of consulting the Catalan people it can. At the moment, the Catalan regional government is the one with the moral and democratic legitimacy to act. If Spain continues to refuse to allow for a legal referendum, then unilateral secession is fully justified. If this were not the case, and that as long as Spain refuses to allow a legal referendum, Catalunya cannot have independence, then you are denying the right of the Catalan people to self-determine.
If Spain refuses to hold a referendum, Catalunya declares unilateral secession, and Spain sends in troops, I would personally do everything in my power to go to Catalunya to do what I could to assist in their favour. States are products of and derive their power from their people; they govern by the assent of the governed. The party at fault here is the Spanish state for not simply allowing a referendum that in all probability they would have won, and handily. It's not enough I get to praise my country's politics (either of them), but on this rare occasion the Scottish example is a clear paradigm for what ought to be done.