This doesn’t seem to contain any information relevant to the legality of voting. It merely discusses the validity of the referendum, which is a separate issue.
I don't understand your logic. Could you try to explain this using common legal concepts?
The constitution merely invalidates the referendum, it doesn't actually make it illegal. However, obviously it's criminal to spend public money on organizing an unrecognized referendum.
Given that the referendum itself isn't criminalized, it seems ridiculous to argue that simply by participating one would be violating the law.
It's also important to keep in mind that even the Spanish government has not attempted to argue that it's illegal to vote. This is just a meme being spread on online forums by people who mostly don't even understand the difference between the constitution and the criminal law.
Nothing about (the English translation of) that sentence supports what you keep asserting. If a Spanish high court decided that's what it means in a decision, that's fine, but you should probably link people to the text of their decision instead of repeating the phrase over and over again as if it's magical.
It still certainly doesn’t make it illegal to vote. This constitution merely tells you what’s a valid referendum and what isn’t. It’s not against the law to participate in an “invalid” one.
It's in the article 1.2 of the Spanish Constitution:
> La soberanía nacional reside en el pueblo español, del que emanan los poderes del Estado.
(The national sovereignty resides in the Spanish people, from which emanate the powers of the State.)