I have a few questions
2)If two catechumens (unbaptized) enter into a civil marriage together before baptism, is a valid natural marriage contracted, or are they just living in sin?
3) How Does one invoke the Pauline or Petrine privlidges? (I read the last answer you gave on this topic. I am however more interested in the canonical procedure for invoking it. For example can I assume that you need a decree from your local ordinary to invoke it? Or would a simple civil divorce in either case do? What is the proper procedure for doing this under cannon law?
Your first question is intriguing. I've consulted my sources and I can't find a good answer. On the one hand, if two unbaptized people get married in a civil ceremony, the church would recognize that as a valid natural marriage. But if they are both catechumens, that means they have made a commitment to become baptized Catholics. I think they would still be validly married, but it begs the question as to why they did not want to wait until they had entered the Church. Certainly later on they can have their marriage "sacramentalized" if they both become baptized Catholics.
The Pauline and Petrine privilege need to be initiated in the marriage tribunal of the diocese. The diocesan tribunal will then analyze the situation and decide whether either privilege can be invoked. It does not automatically happen after a civil divorce.