Right now the idea is that anyone living in Catalonia is a catalan (everyone living in Catalonia has full rights now, and would have them in the event of a vote or independence) and they would have the right to vote in a referendum for independence. Basically anyone who can vote in a catalan autonomic election would be allowed to vote in the referendum. Spanish and catalan are not mutually exclusive concepts. There are no settlers, any spanish citizen living in catalonia has full rights and would have full rights in a hypothetical independent catalonia.
As you said there have been some people saying that all spaniards in Spain, should vote on such a referendum, since the constitution says something to that effect. But honestly I think that is extremely unlikely, even more than a hypothetical referendum only in Catalonia. The government says it as an excuse to avoid a referendum but I think actually doing a contrywide referendum for a regions autonomy would be seen as a joke by outsiders.
In the unlikely event of independence I think the Spanish government will ask spanish citizens living in Catalonia to choose wether they want to be catalan or spanish mainly for practical reasons. Since we have a public pension system (already struggling) if Spain looses all the tax income from Catalonia, from a financial standpoint allowing catalans double nationality would likely be impossible.
As for spaniards moving to Catalonia and becoming catalans, I wouldn't have a problem with those who have at one point in time been catalan (as in they lived here and they learned the language) to come back if they wanted to, I don't think someone who never lived here, or someone who lived here but wasn't willing to learn catalan should automatically become a catalan after independence.
As far as being wanted goes, catalonia is wanted for it's wealth but not for it's catalan identity. For example all catalan public workers speak both catalan and spanish (it's a requirement), however we there are public positions that depend on the spanish government and are not requiered to know catalan. Judges for instance might not know catalan, so while I am allowed by law to address to judges in Catalonia in catalan both in writting and speaking, what use is this law to me if the judge doesn't understand me. Obivously in this case I will have chose to use spanish to communicate with the judge. Or what if I am tried by a catalan speaking judge and my first level appeal (still inside catalonia) goes to a spanish speaking judge who doesn't speak catalan, maybe any paperwork I bring to a courthouse should be in spanish just in case.
The main problem in Catalonia is that while we supposedly have "autonomy" almost any law passed by the regional parliament can be struck down by the spanish parliament, or the constitutional court which nowadays is just a political court isntead of defending constitutinal rights. A "simple" solution which many would be in favour of would be having a federal state inside Spain with clear cut federal and regional jurisdictions, instead of having regional "autonomy" that can be superseded at any time granted by the spanish parliament. But for a federal state you need both sides (Spain and Catalonia) to agree, and so far Spain has never been willing.